| The interaction between HEC and MSS 1 , the seventh regulatory subunit of the 26 S proteasome , was further demonstrated by in vitro GST pull down assays . ^^^ HEC is not a part of the 26 S proteasome and interacts with MSS 1 only when it is dissociated from the complex during M phase . ^^^ Purified MSS 1 specifically hydrolyzes ATP , an activity inhibited by HEC . ^^^ These results show that HEC is a negative regulator of MSS 1 and suggest that it may modulate M phase progression , in part , through the regulation of proteasome mediated degradation of cell cycle regulatory proteins . . ^^^ |