Two conserved serines in the nuclear localization signal flanking region are involved in the nuclear targeting of human lamin A.

The nuclear lamins are karyophilic proteins located at the nucleoplasmic surface of the inner nuclear membrane. We have constructed mutants immediately N-terminal to the nuclear localization signal of human lamin A to identify sites regulating the nuclear transport of the protein. Using an in vitro transport assay, we determined the short-term kinetics of nucleocytoplasmic transport of wild type and mutant proteins. The double mutation of two putative protein kinase C sites (serine 403/404-->alanine) reduced the rate of nuclear import for the mutant protein. Inhibition of phosphorylation in wild type lamin A by the specific protein kinase C inhibitors 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7) and staurosporine or treatment with acid or alkaline phosphatase decreased the nuclear import of the protein. We suggest that transport of human lamin A into the nucleus is regulated by phosphorylations of protein kinase C sites in the sequence N-terminal to the nuclear localization signal.