Identification of nuclear β(II) protein kinase C as a mitotic lamin kinase

Multisite phosphorylation of the nuclear lamins is thought to regulate the process of mitotic nuclear envelope breakdown in vivo. Here we investigate the involvement of two proposed human mitotic lamin kinases, β(II) protein kinase C (PKC) and p34(cdc2)/cyclin B kinase, in human lamin B₁ phosphorylation in vitro and in intact cells. We find that both kinases can phosphorylate purified soluble lamin B at similar rates. However, β(II) PKC phosphorylates interphase nuclear envelope lamin B at more than 200 times the rate of human p34(cdc2)/cyclin B kinase. β(II) PKC-mediated phosphorylation of lamin B is confined to two sites, Ser³⁹⁵ and Ser⁴⁰⁵, within the carboxyl-terminal domain, whereas human p34(cdc2)/cyclin B kinase phosphorylates a single site, Ser²³, in the amino-terminal domain. A second potential p34(cdc2)/cyclin B kinase site within the carboxyl-terminal domain, Ser³⁹³, is not phosphorylated by human p34(cdc2)/cyclin B kinase. However, invertebrate p34(cdc2)/cyclin B kinase from sea star exhibits a different specificity, phosphorylating both amino- and carboxyl-terminal sites. Mitotic human lamin B from intact cells is phosphorylated predominantly in its carboxyl-terminal domain. Comparative tryptic phosphopeptide mapping demonstrates that the β(II) PKC site, Ser⁴⁰⁵, is a prominent target of mitotic lamin B phosphorylation in vivo. β(II) PKC translocates to the nucleus during the G₂/M phase of cell cycle concomitant with phosphorylation of Ser⁴⁰⁵, indicating a physiologic role for nuclear β(II) PKC activation in mitotic lamin B phosphorylation in vivo. The presence of phosphorylation sites within the carboxyl-terminal domain of mitotic lamin B which are not phosphorylated by either β(II) PKC or p34(cdc2)/cyclin B kinase suggests the involvement of other lamin kinase(s) in G₂/M phase lamin B phosphorylation.