MITOTIC PHOSPHORYLATION OF THE LAMIN-B RECEPTOR BY A SERINE ARGININE KINASE AND P34(CDC2)/

The lamin B receptor (LBR) is an integral protein of the inner nuclear membrane that is modified at interphase by a nuclear envelope-bound p rotein kinase. This enzyme (RS kinase) specifically phosphorylates arg inine-serine dipeptide motifs located at the NH2-terminal domain of LB R and regulates its interactions with other nuclear envelope proteins. To compare the phosphorylation state of LBR during interphase and mit osis, we performed phosphopeptide mapping of in vitro and in vivo P-32 -labeled LBR and analyzed a series of recombinant proteins and synthet ic peptides. Our results show that LBR undergoes two types of mitotic phosphorylation mediated by the RS and the p34(cdc2) protein kinases, respectively. The RS kinase modifies similar sites at interphase and m itosis (i.e. Ser(76), Ser(78), Ser(80), Ser(82), Ser(84)), whereas p34 (cdc2) mainly phosphorylates Ser(71), These findings clarify the phosp horylation state of LBR during the cell cycle and provide new informat ion for understanding the mechanisms responsible for nuclear envelope assembly and disassembly.