Ubiquitins
ubiquitins
Ubiquitin is produced by four distinct genes: two of these, UBA52 and RPS27A, encode a single copy fused to ribosomal subunits, whereas the other two, UBB and UBC, are polyubiquitin genes encoding three and nine head-to-tail repeats of ubiquitin, respectively. Free ubiquitin is cleaved from these gene products by peptidases that belong to the deubiquitylating family of enzymes (DUBs). Ubiquitylation principally occurs at lysine residues of substrate proteins, creating an isopeptide bond. Ubiquitination was initially recognised as a signal for proteasome-mediated degradation but is now known to be fundamental to the control of many cellular processes.