In addition to the degradation of mutant, damaged and misfolded proteins, this system is responsible for the modulation of many regulatory proteins such as cyclins [14] , inhibitors of cyclin-dependent kinases (p21, p27) [15] , tumor suppressors (p53) [16] , and inhibitor of NFkB (IkB) [17] , which are essential for a variety of cellular functions, including cell-cycle regulation, apoptosis and host immune responses [18] . Unlike polyubiquitination in the regulation of protein degradation, monoubiquitination of cellular proteins, such as histones, calmodulins, actin, proliferating cell nuclear antigen and receptor tyrosine kinases, plays more diversified roles involving in the regulation of chromatin remodeling, DNA repair, transcriptional regulation and endocytosis [19] .