THE DESTRUCTION BOX OF CYCLIN-A ALLOWS B-TYPE CYCLINS TO BE UBIQUITINATED, BUT NOT EFFICIENTLY DESTROYED

The destruction of mitotic cyclins by programmed proteolysis at the en d of mitosis is an important element in cell cycle control, This prote olysis depends on a conserved motif of nine residues known as the 'des truction box', which is located 40-50 residues from the N-terminus, Th e sequences of the A- and B-type destruction boxes are slightly differ ent, which might account for the differences in timing of their destru ction. When the cyclin A-type destruction box was substituted for the normal one in cyclin B1 or B2, however, the resulting constructs were unexpectedly stable, although the converse substitution of B-type dest ruction boxes in cyclin A permitted normal degradation, We compared th e ubiquitination of various cyclin constructs, and found that whereas mutation of the highly conserved residues in the destruction box stron gly reduced the level of ubiquitinated intermediates, the stable destr uction box 'swap' constructs did form such adducts, Thus, while ubiqui tination is probably necessary for cyclin destruction, it is not suffi cient. We also found that poly-ubiquitinated cyclin derivatives are st ill bound to p34(cdc2), which is not detectably ubiquitinated itself, raising the questions of how cyclin and cdc2 dissociate from one anoth er, and at what stage, in the process of degradation.