A mutant deubiquitinating enzyme (Ubp-M) associates with mitotic chromosomes and blocks cell division

A. new ubiquitin-processing protease (Ubp-M) has been identified in mammali an cells that is phosphorylated at the onset of mitosis and dephosphorylate d during the metaphase/anaphase transition. The carboxyl-terminal domain of this 823-aa protein can be phosphorylated in vitro with either extracts of mitotic cells or purified cdc-2/cyclin B complexes, Recombinant Ubp-M is a ble to deubiquitinate histone H2A in vitro, and the phosphorylated form is also enzymatically active. Wild-type Ubp-M. transiently expressed as green fluorescent protein-fusion proteins, localizes in the cytoplasm of cultured cells, but mutant forms? lacking an active-site cysteine, associate closel y with mitotic chromosomes during all stages of cell division and remain wi thin the nucleus during the postmitotic period. Cells transfected with plas mids containing mutant Ubp-M genes stop dividing and eventually undergo apo ptosis, Ubp-M may deubiquitinate one or more critical proteins that are inv olved in the condensation of mitotic chromosomes, possibly acting selective ly on histones H2A and H2B, the major ubiquitinated proteins of chromatin.