2 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 nucleoplasm ic surface of tile inner nuclear membrane. We have constructed mutants immediately N terminal to the nuclear localization signal of human la min A to identify sites regulating the nuclear transport of the protei n. Using an in vitro transport assay, we determined the shortterm kine tics 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 pr otein, Inhibition of phosphorylation in wild type lamin A by the speci fic protein kinase C inhibitors 1-(5-isoquinolinesulfonyl)-2-methylpip erazine (H7) and staurosporine or treatment with acid or alkaline phos phatase decreased the nuclear import of the protein. We suggest that t ransport of human lamin A into the nucleus is regulated by phosphoryla tions of protein kinase C sites in the sequence N-terminal to the nucl ear localization signal.