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In this essay, we are focusing on the processes of monoubiquitination and its relation to the protein substrate. This draws attention on the regulatory of protein, which is available in almost all cell tissues. It is evident that the Tob family proteins can undergo monoubiquitination in the deficiency of ubiquitin ligase in vitro. Monoubiquitination is the addition of a single ubiquitin moiety to a protein substrate. It can also refer to the adaption of the protein by the covalent attachment of individual or more ubiquitin molecules to the ubiquitin. Ubiquitin is a small protein that is regulatory which found in almost every form tissue of an eukaryotic organism. It also functions to direct the process of protein recycling. The proteins are banded by ubiquitin ready for destruction. The proteins are directed to the proteasome by the ubiquitin tag.

Proteasome is a large protein complex in the cells, which degrades and recycles proteins that are not required. Ubiquitin tags also control other protein and cell mechanisms to the other locations where they direct proteins to take other activities. Ubiquitin receptors that attach ubiquitinated proteins by ubiquitin-binding domains contain key roles in a variety of cellular processes. These receptors are in most cases monoubiquitinated, referred to as couple monoubiquitination. Addition of ubiquitin chains to aims proteins leads to their monoubiquitination. This monoubiquitination aims proteins for degradation also to regulate receptor internalization and endosomal sorting.


Ubiquitin is a 76 amino acid residue that binds covalently to lysine residues in aimed proteins and standardizes their stability. It is the preteosome-dependent degradation of Tob family proteins and is necessary in the control of cell cycle progression as well as DNA damage responses. A variety of Ub ligases referred to as ubiquiting ligases is responsible for degradation of Tob protein. This brings out the fact that Tob family proteins can undergo monoubiquitination in the deficiency of ubiquiting ligases in vitro.

Interpreting the role of mammalian H2B ubiquitation could undergo extensive facilitation by antibodies specific to ubH2B. However, unlike other histone covalent modifications, the ubiquitin moiety is a 76 amino acid polypeptide that cannot be compromised within the specificity determination area of an antibody. In overcoming this obstruction, there should be the use of a branched peptide parallel to the to the conjugating site of ubiquitin on human histone H2B. A series of the ubH2B-specific hybridomas is to be obtained. When the antibody is reacted with a whole cell extract or acid extractof human U2OS cells, it recognises a single band migrating at the position of ubH2B with no detectable cross reactivity with either non-ubiquitinated H2B or ubH2A. Further confirmation can be done through the use of purified calf thymus histone H2B, in addition of the acid extract, fraction from HEK293 cells. Moreover, the antibody identifies its target in the extracts of cells transfected with wild-type human H2B, and not with H2B mutated at Lys 120 that is not able to undergo monoubiquitation. Remarkably, the similar band is as well identified in though in recessive quantity with an anti-ubiquitin antibody. Saccharomyces cerevisiae ubH2B, however, is not recognised efficiently.

The covalent accessory of the diminutive proteins ubiquitin to a lysine remains in a substrate protein occurs in three crucial steps. The sequential action of ubiquitin activating is the first process through which the protein ubiquitination occurs. Conjugation of the protein substrate to lysine is the second step and finally ligase of the enzymes. Because of versatility, ubiquitination acts to regulate a variety of cellular processes including signal transduction, protein degradation, DNA repair, chromatin remodelling, and membrane traffic and peroxisome biogenesis. Ubiquitination may regulate the functioning of the protein through conformational changes. This is by ubiquitin receptors that are acquainted with the ubiquitinated protein and there for control downstream biochemical processes.

Multi-ubiquitin chains, composed of at least four sub-units, is essential to ensure efficiency in recognition and degradation of ubiquitylated proteins by the proteosome. Nevertheless, other functions of ubiquitin have been discovered that do not engage the proteaosome. A solitary ubiquitin or short ubiquitin chains alters some proteins. Instead of sending the proteins to their death via the proteaosome, monoubiquitylation adjusts the processes that vary from membrane transport to transcriptional regulation.

Independent ubiquitination in vitro

The in vitro ubiquitination essay essentially describes the modifications of the protein. The polyubiquitination aims at destroying the proteasome. Mutational dysfunction of PARKIN gene that encodes a double ring contains ligase E3 activity. This is the foremost cause the autosomal recessive juvenile parkinsonism. The rear ring finger motif is responsible for the Parkin E3 activity (Khanna & Shiloh, 2009). MBP-parkin shows higher molecular weight from self ubiquitination in full reaction mixtures containing either full length parking, or IBR-R2 but not parkin C44R RING2 mutant. The parkin species are not detected especially when E1 or parkin is omitted.

The HMW parkin species are not detected when E1 or the parkin is omitted, and in the absence of E2, both the MBP-parkin and IBR-R2 appear capable of the mediating autobiquitination. FK1 immunoblotting that specifically identifies species produces a laddering pattern in MBP-parkin as well as IBR-R2 reactions that are catalyzed in the absence or presence of E2. Although ubiquitinated species are produced by IBR-R2, they tend to be of a higher molecular weight than those that are produced by the full-length protein. FK2 immunoblotting that identifies both mono- and polyubiquitinated proteins it also reveals a laddering pattern that is similar to FK1. When FK2 and FK1 are compared side by side, immunoreactivities can be significantly being much robust in reactions catalyzed by the IBR-R2 than those catalyzed by the full-length protein.

There is no FK1 or FK2 immunoreactivity that is detected in reactions that are conducted with parkin C441R mutant. Parkin-mediated ubiquitination in vitro can occur in the presence or absence of E2 and the nature of ubiquitinationcatalyzed by full-length parkin and IBR-R2 differs.

The numerous forms of parkin-mediated ubiquitination can most probably endow the protein with a bigger elasticity to react to varying cellular conditions. For instance, upon mitochondrial depolarization, parkin is recognized to modify proteins on the dented organelles through a variety of ubiquitin linkages, including K27, K48 and/or 65. In addition, during times of proteasomal stress, parkin-mediated K63. Ubiquitination can be favoured over the ability to redirect protein weight away from an otherwise besieged proteasome. Ubiquitin contains several lysine residues, which can serve as points of Polyubiquitylation which are: K48, K63, K6, K11, K27, K29, and K33. The different linkages may define various signals that are identified by the ubiquitin-binding proteins. The proteins have Ubiquitin interacting motifs, which bind to the ubiquitin. Ubiquitin is a 76 amino acid residue that binds covalently to lysine residues in aimed proteins and standardizes their stability. It is the preteosome-dependent degradation of Tob family proteins and is necessary in the control of cell cycle progression as well as DNA damage responses.

A variety of Ub ligases referred to as ubiquiting ligases is responsible for degradation of Tob protein. This brings out the fact that Tob family proteins can undergo monoubiquitination in the deficiency of ubiquiting ligases in vitro. Ubiquitin is a miniature regulatory protein that is present in nearly all tissues (ubiquitously), in eukaryotic organisms. It acts as the overall protein recycler among other functions. It binds with proteins and marks them for destruction. The tag acts as a director of proteins to the proteosome that is a large protein complex in the cell responsible for degrading and regenerating unneeded proteins.

The tag can as well direct proteins to other sites in within the cell for the control of different protein and cell mechanism. Marking of Trans membrane proteins for elimination from membranes is also another job of ubiquitin and performs a series of benchmark roles in the cell. A good example of these proteins is the receptors. Monoubiquitination is done to cell-surface transmembrane molecules that ubiquitin binds. This adjustment alters the protein’s sub cellular localization often aiming at the protein for impairment in lysosomes. Protein ubiquitination controls the half-lives of many proteins through marking them for degradation. Ubiqutination is a definite system linked with a number of highly synchronized biological results including cell differentiation, progression, retrovirus assembly, antigen presentation, apoptosis, signal transduction, biological clocks, transcriptional activation, receptor down regulation, as well as endocytosis.

Recently discovered families of ubiquitination and deubiquitination enzymes take part in processes such as these. They could provide new families of drug awareness and new ways of intervention in human diseases. Conversely, much work is required to confirm this approach. A covalent linkage to Ub characterizes proteins marked for degradation by the Ub preteosome system. The activating enzyme (ubiquiting activating enzyme) activates Ub initially, in a manner subsidiary of ATP. After the activation, Ub is transferred subsequently to a conjugating enzyme (ubiquiting conjugating enzyme) that binds it eventually to a lysine residue on the substrate through a process refereed by ubiquiting ligase (Peng & Schwartz, 2003).

There are varieties of Ub modifications with different cellular roles. A poly Ub chain linked to Lys48 is allied to proteosome-dependent degradation while that linked to Lys63 affects DNA repair processes and endocytosis. Furthermore, monoubiquitination, which involves covalent modifications of proteins by a single Ub, plays a crucial role in regulation of signal transduction, endocytosis, DNA repair processes, histone activity, and retroviral budding. Recent research shows that a series of Ub binding domain (UBD) containing proteins to be monoubiquitinated in vitro autonomously of the activity of ubiquiting ligases. When ubiquitin-binding proteins are monoubiquitinated, their capacity to bind and control the functions of ubiquitinated targets in vivo is inhibited. Though, as earlier shown the Tob/ BTG proteins are monoubiquitinated in the absence of ubiquiting ligases in vitro. Polyubiquitination and monoubiquitination have different roles. While monoubiquitination is thought to be the regulator of receptor internalization as well as endosomal sorting, polyubiquitination is responsible for finding proteins for degradation.

Further, monoubiquitination occurrence associates with minor levels of polyubiquitination. As a result, monoubiquitination of Tob/BTG family proteins tends to adjust their stability by affecting polyubiquitination, as well as concomitant proteosome-dependent degradation. Proteins in the Tob/BTG family are polyubiquitinated and eventually degraded by the 26S proteosome. In this process, a number of E3 Ub ligases are accountable for Tob ubiquitination. In the detection of ubiquitination in Tob, a solitary slow migrating form of Tob migration confirms it through immunoblotting even in reactions that lack cytoplasmic extracts. An anti-Ub antibody that is produced when there is the presence of all protein components recognizes this. Thus, we learn that Tob is monoubiquitinised in vitro in an E3s independent manner. The amino acid sequence (amino acids 1-112) is well conserved in the Tob/BTG family proteins. In testing whether the tendency of undergoing monoubiquintination is conserved among protein family members, results show that both Tob2 family and BTG2 become monoubiquitinated, as well. The mediators of the monoubiquitination in vitro were UBCH4 and UBCH5 but not UBCH3 interestingly. Determination of the sites of monoubiquitination is done through the process of mass spectrometry. Here, large amounts of in vitro ubiquitinated Tob proteins are subjected to SDS-PAGE. The gels are silver stained and the corresponding cut out from the gel.

Peptide mass fingerprints then are used to examine this. There is the identification of Lys48 and Lys63 of the Tob in vitro monoubiquitinated sites. The results show that the monoubiquitination of Tob occurs within its terminal half. It is also worth noting that the amino acid sequence around Lys48 and Lys63 of Tob, are located in a domain that is highly conserved among Tob/BTG family proteins. Tob is ubiquitinated at Lys48 and Lys68 in vitro. For confirmation, Tob mutants are constructed in which Lys48, Lys63 or both are replaced with alanine (TobK48A, TobK63A, TobK48, 63A correspondingly). A slow-migrating form from TobK48A mutant is produced in small amounts in vitro ubiquitination. In the reaction products of the TobK63A mutant, a slow form migrating was seen too though in somewhat lesser amounts than that in the wild-type Tob. TobK48, 63A had no steady-migrating form. This suggests that Lys48 is a leading monoubiquitination site in Tob. It also shows that the contribution of Lys63 in monoubiquitination is small. This shows that Lys48 and not Lys63 are conserved in among the ensuing amino acid chains of all Tob/BTG proteins. The biological significance of Tob monoubiquitination is examined through transfecting wild-type Tob or Tob mutant into Cos 7 cells alongside 6xHis-tagged Ub expression vector. His-Ub-conjugated proteins are then purified with Ni-NTA agarose under denaturing conditions.

The results of the immunoblot analysis show that polyubiquitination of TobK48A and TobK63A is enhanced by the relatively to wild-type Tob. The meaning of this is that monoubiquitination of Tob at Lys48 or Lys63 holds polyubiquitination of Tob in vivo. It is worth noting that the level of polyubiquitination of TobK63A is higher than that of TobK48A. Though Lys48 is a key Tob ubiquitination site in vitro, monoubiquitination at Lys48 contributes more sturdily than that at Lys48 towards discretion of Tob polyubiquitination in vivo. Nonetheless, since TobK48, 63A is more polyubiquitinated than either TobK48A or TobK63A, monoubiquitination at both Lys48 and Lys63 down-regulated polyubiquitination of Tob.

From all this tests, we find out that the Tob/BTG family proteins experience monoubiquitination in the absence of E3s in vitro. The major cites of monoubiquitination are found to be the lysine residues that are universally conserved among members. Monoubiquitination suppresses polyubiquitination somehow viewing from the elevated polyubiquitination of TobK48, 63A comparing to that of wild-type Tob. Similarly, as monoubiquitination of UBD-containing proteins activates intramolecular exchanges with the UBD that prevents them from binding in Trans to ubiquitinated targets, monoubiquitination of Tob may induce a conformational change in that it hampers E3s’ access, ensuing in the containment of polyubiquitination. On the other hand, UBD containing proteins may well be involved (Peng & Schwartz, 2003). Given that the UBD containing proteins identifies monoubiquinated Tob, they could prevent E3s from polyubiquitinating the monoubiquitinated Tob proteins. Analyzing the monoubiquitinated Tob structurally will be of much help in understanding the inhibition of polyubiquitination by monoubiquitination’s cause is by intermolecular or intermolecular relations.

In the latter case, identifying molecules that exclusively interact with monoubiquitinated Tob would be an exercise of much significance. As the UBD containing proteins monoubiquitinate themselves through the interaction between UBD and Ub-conjugated E2 enzymes, there is the lack of any obvious UBD in the Tob/BTG family. Even so, we are not justified to exclude the likelihood that the exclusive sequences conserved amongst the proteins of the Tob/BTG family in their amino terminal regions might as well serve as a novel field for self-monoubiquitination. Tob is conscientious for the keeping of cells in the quiescent stage in the absence of mitogenic stimuli as well as blocking DNA damage-induced apoptosis. Monoubiquitination of Tob might modulate Tob’s ability to perform these actions.

Generation of antibodies specific to monoubiquitinated Tob is required to comprehend its sub cellular localization, and protein interactions and in due course realize the biological significance of Tob moubiquitination. Conclusively, the current findings that the degree of polyubiquitination is controlled by its monoubiqutination indicate a novel strategy for proteins stabilization. Cbl proteins are as well chief ubiquitin ligases that promote ligand-dependent polyubiquitination in line with degradation of receptor tyrosine kinases. They control receptor endocytosis by recruiting CIN85-endophilin in the compound with activated receptors. This is achieved through the monoubiquitination of the adaptor protein CIN85 and its homologue CMS by Cal/Cbl-b following epidermal growth factor (EGH) stimulation.

However, monoubiquitination of CIN85 needs direct interactions between CIN85 and Cbl, the integral ring finger domain of Cbl and an ubiquitin acceptor site current in the carboxyl terminus of CIN85. Cbl-b and monoubiquitinated CIN85 are present in the complex alongside polyubiquitinated EGF receptors during prolonged EGF stimulation and are degraded jointly in the lysosome. This shows that Cbl/Cbl-b can mediate the polyubiquitination of cargo on top of monoubiquitination of CIN85 in the intent of controlling endosomal cataloging and degradation of receptor tyrosine kinases (Khanna & Shiloh, 2009).

Protein ubiquitination controls the half-lives of many proteins through marking them for degradation. Ubiqutination is a definite system linked with a number of highly synchronized biological results including cell differentiation, progression, retrovirus assembly, antigen presentation, apoptosis, signal transduction, biological clocks, transcriptional activation, receptor down regulation, as well as endocytosis. A number of proteins contain ubiquitin-binding domains otherwise referred to as motifs such as the UIM (ubiquitin-interacting motif). They are called ubiquitin receptors and are often monoubiquitinated through a procedure requiring the presence of a UBD. This is referred to as coupled monoubiquitination.

 Recently discovered families of ubiquitination and deubiquitination enzymes take part in processes such as these. A number of proteins contain ubiquitin-binding domains otherwise referred to as motifs such as the UIM (ubiquitin-interacting motif). They are called ubiquitin receptors and are often monoubiquitinated through a procedure requiring the presence of a UBD. This is referred to as coupled monoubiquitination. Coupled monoubiquitination highly relies on the ability of UIM in binding to monoubiquitin (mUb). Here, the fundamental, molecular mechanism is based on the interface between the UIM and ubiquitin ligase E3 that has already been already undergone modification through ubiquitination. Further, the in vivo ubiquitination of E3 ligases is associates with their capacity to monoubiquitinate eps15 (endocytic protein). Thus, the ubiquitination E3 ligases play a vital part in the process of coupling monoubiquitination.

Monoubiquitination in Receptor Internalization and Endocytosis

This is the Ligand-induced ubiquitination of the membrane receptors that include receptor tyrosine kinases, which are concerned in receptor internalization and endocytosis. Monoubiquitination of the G-protein coupled receptor in the yeast leads to its internalization and consequently degradation in the vacuole of the yeast. Other transmembrane such as permeases, transports also go internalization upon monoubiquitination. In the mammalian cell, the transmembrane receptors and the endocytic adaptor proteins move through ubiquitination upon stimulation with ligand. EGF-induced internalization and organizing to lysosomes of the epidermal growth factor (EGFR) is an outstanding example of how Ub regulates receptor degradation.

Protein chimera that contains a sole Ub moiety compound to the cytoplasmic tail of the EGFR is also internalised from the cell surface. It is also targeted to late endosomal compartments for degradation. Monoubiquitin can serve as both internalization as well as a sorting signal. EFGR is a mult-monouqutinated also polyubiquitinated by Lys63-linked chains. Although mono Ub is sufficient for internalization as yeast, polyubiquitination by Lys63 generates a more effective internalization signal increasing binding avidity.

Monoubiquitination of Ub-receptors

Several receptors have the ability to bind the Ub receptors and at the same time they can undergo monoubiquitination upon ligand stimulation, the UBDs of Bb- receptors are eminent in their monoubiquitination. Those kinds of Ub-receptors undergo monoubiquitination through a molecular process known as coupled monoubiquitination. The process needs the presence of an intact UBD. The underlying molecular process of the coupled monoubiquitination is eradicated using the Eps 15 adaptor protein. The adaptor protein undergoes EGF-induced.

Monoubiquitination as a model system. The processes involve the interaction between the UIM of the Ub-receptor and a HECT-type E3 ligase that already has been modified by ubiquitination. Ubiquitin receptors that combine ubiquitinated proteins through ubiquitin binding realm have key roles in various cellular processes. These types of receptors are often themselves monoubiquitinated in a process referred to as coupled monoubiquitination. This process has been proven to involve monoubiquitination of an ubiquitin ligase and its ensuing interface with an ubiquitin receptor.

The modified E3 ligase then moves the thiolester-conjugated Ub from its catalytic cysteine residue up to the Ub-receptor. The Ub-receptor then becomes.

Monoubiquitinated. Monoubiquitination of Ub-receptors plays also the regulatory role in determining their binding capacity to ubiquitinated loads. Ubiquitination reaction products that are generated by MBP proteins containing the catalytic ring domain of various E3 members that are tested. That is in the presence or absence of their cognate E2 and presence of UbcH7 is noted. Addition of a single ubiquitin moiety to a substrate of protein yields a polyubiquitin chain. Some substrates are modified by the addition of the molecules of ubiquitin to multiple lysine residues in a process called multiubiquitination. Ubiquitin contains a total of seven lysine residues. Several ubiquitin receptors that include proteins that control endocytic membrane traffic are monoubiquitiated. This type of modification is called coupled monoubiquitination and requires the presence of a UBD to sustain this process. Coupled monoubiquitination is likely to exert a negative feedback control to in the molecular machineries. This is especially to machineries that need transient and successive interactions between the ubiquitin and their UBDs and thus becoming auto inhibited.

The endocytic membrane trafficking that involves complex machinery encompassing ubiquitin receptors represents an outstanding example of such regulation. This mechanism also applies the DNA repair where the localization of translation polymerases out or in replication factories is controlled by their binding ability. Monoubiquitination of an ubiquitin lagase and its interaction with the UBD in the ubiquitin receptor is extremely significant to the receptor`s coupled monoubiquitination (Peng & Schwartz, 2003).

Mutations in the parkin gene are a prime cause of the autosomal recessive in early-onset Parkinson. Parkin functions as an ubiquitin ligase (E3) that is associated with the ubiquitin-proteasome system (UPS), key intracellular proteolytic machinery, which works to destroy the unwanted proteins in the cell. Parkin when is an E3 member it works in association with other vital members. The members that work in associate with parkin are ubiquitin activating (E1) and the conjugating (E2) enzymes. The parkins work to catalyze the formation of the ubiquitin chain on its substrates, which acts as a targeting indication meant for proteasome-mediated degradation. Ligination reaction associated with ubiquitin –mediated protein degradation takes place. The degradation takes place between the terminal residue (G76) of a single ubiquitin molecule and the internal lysine (K) residue at point 48 within another. It is also possible that the ubiquitin chain assembly can also take place in molecules such as K63.

In an account, to that it is also evident that proteins can also be monoubiquitinated (6, 7). These kinds of non-canonical modification in most cases serve up as non-proteolytic signals. These non-proteolytic are involved in a variety of cellular process that includes the DNA repair and endocytosis (7). Parkin is associated with protein degradation, but it is a unique E3 capable of mediating monoubiquitinated the same as K63-linked polyubiquitination (8, 9, 10, and 11). However, a single enzyme can exhibit such multifunctional properties. It is of also of immense importance to note that the choice of E2 partners may influence the topology of the ubiquitin sequence assembly mediated by parkin (Khanna & Shiloh, 2009).

The parkin interaction with the heterodimeric Ubc13/Uev la E2 pair seems to errand K63-linked polyubiquitination. But its partnership with the UbcH17 promotes K48-linked polyubiquitination (8 and 12). Determinants that regulate parkin`s choice of E2, which are not commonly known but study indicates that parkin phosphorylation by PINK 1 enhances its recruitment of Ubc 13 (13). The differential recruitment of the E2s cannot adequately explain the monoubiquitination activity of the parkin.

Parkin is also unique E3 that it is capable of mediating ubiquitination in an E2 independent way, which is a novel activity by an E3 member that is unprecedented. It is particularly appealing to take into account that whereas full length parkin catalyzes E2 monobiquitination in vitro, a truncated mutant which contains only C-terminal IBR-R2 region catalyzes both mono and polyubiquitination in the absence of E2. Presence of K48 linked polyubiquitin in reaction products that are catalyzed by IBR-R2 but not that of full-length parkin. Significantly, the ubiquitin chains that are formed by the IBR-R2 become modified in the existence of E2s. For instance, mixed chains of K48 and K63 linked ubiquitin polymers are produced when the UbcH7 is replaced by Ubc13/Uev la. There are no such kinds of modifications that occur in the reactions that contain the full length parkin.

The role of TSG101 in identifying monoubiquitinated targets is determined by the ubiquitination of the of the LDH enzymes. In finding this out,  LDH-A as well as LDH-B are over expressed with HA –tagged ubiquitin in Cos-1 cells under normal oxygen conditions. Ensuing growth in normal oxygen conditions, both LDH-A and LDH-B are expressed at similar levels at approximately 38kDa. Parallel western blot analysis for the incorporation of the ubiquitin in LDH enzymes reveals that only the anaerobic isoform, LDH-A, ought to be ubiquitited.

Previous research in the past decade tells about a convergence of the otherwise distinct regions of the of DNA replication, recombination and makeup, as we are learning how these vital connections can function in coordination to reach a genomic stability as well as guarantee cellular variability. In the next 10 years, we can expect a functional understanding in the roles of the posttranslational protein modifications in the regulation alongside prioritizing of pathways in genomic maintenance. The fundamental knowledge gained through this research ought to give the right direction towards extensively efficient clinical intervention in human diseases. This can play a crucial role in prevention and disease control let alone curing. It can as well help in learning more on genetic diseases that for a long time have been known to be of less curability.

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