an inactive trimer with APC and CDC20, forming the hMAD2-CDC-APC complex. Upon the ubiquitination of CDC20 by UbcH10, hMAD2 dissociates, and APC/C becomes active. It is important to note that ubiquitination of CDC20 does not serve to mark it for degradation, but rather promote dissociation of hMAD2 from the hMAD2-CDC-APC complex. USP44, a ubiquitin-specific-processing protease, can stabilize the inactive hMAD2-CDC-APC complex by counteracting UbcH10 ubiquitination. This blocks hMAD2 dissociation and allows for proper regulation of APC/C, keeping it inactive until proper attachment of the mitotic spindle. Upon proper attachment, switch-like behavior allows for the activation of APC/C. This results in the cleavage of cohesion, allowing for the separation of sister chromatids. Role in p53-mediated DNA Damage Repair DNA damage can prove catastrophic for an organism. Mechanisms for DNA mutation include oxidative stress, DNA replication errors, exogenous carcinogens, radiation, and spontaneous base mutation. Upon DNA damage, cell cycle progression is halted to prevent propagation of the mutation. The TP53 gene (also known as p53) is crucial in ensuring the conservation of the genome. Deubiquitinating enzymes play an integral role in maintaining p53’s function. In healthy cells, p53 activates the E3 ubiquitin ligase MDM2 which in turn ubiquitinates p53. This creates a negative feedback loop, whereby the degradation of p53 allows for cells to flow through the cell cycle. Upon DNA damage, Ubiquitin-specific-processing protease 7 (USP7) stabilizes p53 by cleaving ubiquitin. For USP7 to deubiquitinate p53, it must localize to the nucleus. However, no nuclear localization sequence (NLS) has been found. Despite no known NLS, one study showed that, upon deletion of USP7’s N-terminus, no nuclear localization occurred. It is possible that other proteins facilitate nuclear entry of USP7. Once stabilized, p53 can exert its tumor suppression function. Downstream pathways of p53 act to either halt cell cycle progression in G1 or G2 phases of the cell cycle or promote cell-death, depending on the severity of the DNA damage. See schematic of the role of USP7 in the p53-dependent pathway. or promote cell-death, depending on the severity of the DNA damage. See schematic of the role of USP7 in the p53-dependent pathway. References Protein domains Enzymes