HEC BINDS TO THE 7TH REGULATORY SUBUNIT OF THE 26 S PROTEASOME AND MODULATES THE PROTEOLYSIS OF MITOTIC CYCLINS

A newly identified nuclear protein rich in leucine heptad repeats call ed HEC is important for mitosis. To elucidate its mechanism of action, the region containing leucine heptad repeats was used to identify cel lular proteins that potentially interact with HEC. Complementary DNAs encoding several proteins including MSS1, p45, Nek2, and Smc1/Smc2, kn own to be important for G(2)/M progression, were identified. The inter action be tween HEC and MSS1, the seventh regulatory subunit of the 26 S proteasome, was further demonstrated by in vitro GST pull-down assa ys. HEC is not a part of the 26 S proteasome and interacts with MSS1 o nly when it is dissociated from the complex during M phase. Purified M SS1 specifically hydrolyzes ATP, an activity inhibited by HEC. In addi tion, HEC inhibits the proteolysis of mitotic cyclin B in vitro. Consi stent with this biochemical activity, ectopic expression of HEC inhibi ts the degradation of mitotic cyclins after telophase, resulting event ually in cell death. These results show that HEC is a negative regulat or of MSS1 and suggest that it may modulate M phase progression, in pa rt, through the regulation of proteasome mediated degradation of cell cycle regulatory proteins.