Identification of novel phosphorylation sites in murine A‐type lamins

We report the distribution of phosphorylation sites in murine lamins A and C (A‐type lamins) in vitro and in vivo followed by reverse‐phase high‐performance liquid chromatography and micro‐sequencing of peptides spanning the almost complete lamin sequence. We show that two distinct protein kinases, cell‐division‐cycle‐2 kinase (cdc2 kinase) and protein kinase C (PKC), phosphorylate murine A‐type lamins at the non‐α‐helical amino‐ and carboxy‐terminal domains in vitro and in vivo. Cdc2 kinase, but not PKC, is capable of inducing depolymerization of the nuclear lamina in permeabilized cells. Accordingly, lamins were proposed to be direct in vivo substrates of cdc2 kinase and PKC with different effects on the lamina dynamics. Analysis of the original A‐type lamins revealed phosphorylation of residues Ser5 and Ser392. Residue Ser392 was substoichiometrically phosphorylated in the substrate and by cdc2 kinase in vitro. PKC phosphorylated peptides with its kinase‐specific motifs surrounding Ser5, Thr199, Thr416, Thr480 and Ser625. In vivo, a mitosis‐specific phosphorylation at the cdc2‐kinase‐specific phosphoacceptor site Ser392 and of the N‐terminal peptide was identified. An interphase‐specific phosphorylation at Ser525 matching the PKC consensus sequence and of peptides phosphorylated by unknown kinases was determined. The results lead us to propose that different cyclin‐dependent kinase activities act as lamin kinases in mitosis and in interphase. Other kinases may cooperate with cdc2 kinase during reversible disassembly in mitosis and may modulate the supramolecular assembly of lamin filaments.