br Ubiquitylation of the TGF BMP pathway components Ubiquity

Ubiquitylation of the TGFβ/BMP pathway components Ubiquitylation is a reversible post-translational modification that is essential in many cellular regulatory mechanisms [20], [21]. During the ubiquitylation cascade, ubiquitin is attached to target proteins through the concerted actions of an E1-ubiquitin activating enzyme, an E2-ubiquitin conjugating enzyme and an E3-ubiquitin ligase. This cascade is initiated by the ATP dependent activation of ubiquitin by the E1. The E1 links the C-terminal glycine residue of ubiquitin via a thioester bond to a cysteine residue within its active site. The activated ubiquitin intermediate is then transferred to a cysteine residue of the E2 enzyme. In the human genome, there are ~45 different E2 enzymes, which determine the type of the ubiquitin chain linkage that is assembled on the substrate. The ubiquitin-loaded E2 then pairs with specific E3-ubiquitin ligases, which facilitates the conjugation of C-terminal glycine of ubiquitin via an isopeptide bond to the ε-amino group of the target lysine on the substrate protein, which can be ubiquitin itself. More than 600 E3 ubiquitin ligases are encoded in the human genome, and in certain cases their associated substrate adaptor proteins often recruit substrates, thereby providing specificity in the ubiquitylation process [22], [23], [24], [25], [26]. Target proteins can be monoubiquitylated, multi-monoubiquitylated or by repeated action of the E1, E2 and E3 ligases ubiquitin can be added onto one of several lysine residues or the α-amino group of the attached ubiquitin to form unique polyubiquitin chains. The nature of ubiquitylation on the target protein defines its fate, from altered subcellular localisation or activity to destruction through the proteasome or lysosome [27], [28], [29]. The E3 ubiquitin ligases involved in the ubiquitylation of TGFβ pathway components have been reviewed extensively [14], [15], [17], [30], [31], [32] (Fig. 1). However, very little is known about the E2 chemicals library that are involved in the ubiquitylation of TGFβ pathway components, with UBE2L3 and UBE2O the only members implicated [33], [34]. Furthermore, the precise ubiquitylation sites on most of the TGFβ pathway components known to be ubiquitylated have not been established at the endogenous level. Other than the K48-linked ubiquitin chains that mostly confer destructive fate on target proteins, there has been a lack of comprehensive linkage-type analyses on polyubiquitin chains on the TGFβ-pathway components. Similarly, whether and how these chains are recognised by potential ubiquitin-binding proteins, that then modulate the fate of the target protein, remain to be defined.
Regulation of the TGFβ/BMP pathway components by deubiquitylating enzymes
Perspective As we have discussed above, various diseases are linked to the malfunction of the TGFβ pathway. The findings that several DUBs target components of the TGFβ and BMP pathways for deubiquitylation to drive normal signalling imply that DUBs could be suitable candidate targets for the development of small molecule inhibitors of the TGFβ and BMP pathways. Selective inhibitors of these DUBs could be potentially useful therapeutically against pathologies associated with abnormal TGFβ and BMP signalling. Furthermore, there are recent reports that imply certain DUBs are themselves modified or mis-expressed in cancers [43] thereby affecting TGFβ signalling. While research projects aiming to develop selective inhibitors of DUBs are underway, caution needs to be taken, as each DUB would be predicted to target many substrates beyond TGFβ/BMP signalling. A better strategy would therefore be to understand the molecular mechanisms by which a DUB is recruited to its target in the TGFβ and BMP pathways. This would allow for the development of small molecules that selectively inhibit the interaction between the DUB and its target in the TGFβ pathway but not other targets. Indeed, the tail phosphorylation-dependent recruitment of OTUB1 to SMAD2/3 is a case for which interaction determinants are known [54].