Nuclear transport kinetics depend on phosphorylation-site-containing sequences flanking the karyophilic signal of the Simian virus 40 T-antigen.

Selective nuclear protein transport was analyzed in single living cells. Hybrid proteins consisting of short stretches of the Simian virus 40 T-antigen and of the almost complete beta-galactosidase moiety were generated by molecular genetic methods and injected into the cytoplasm of rodent hepatoma cells. A histochemical assay showed that all proteins containing the karyophilic signal of the T-antigen (residues 126/127-132) were equally well accumulated by the nucleus within 15 h after injection. Microfluorimetric measurements of nuclear transport kinetics, however, revealed large differences. Proteins containing the karyophilic signal without flanking sequences were taken up by the nucleus on a time scale of hours. The same held for a protein containing T-antigen residues 127-147. However, a protein containing T-antigen residues 111-135 was accumulated by the nucleus with a half-time of 8-10 min reaching an equilibrium nucleocytoplasmic concentration ratio of greater than or equal to 15. Photobleaching measurements showed that, independently of subcellular localization, the mobility of all proteins was quite large. Thus, our measurements revealed a striking effect of T-antigen residues 111-125 on the kinetics of nuclear transport. Residues 111-125 do not seem to contain a second karyophilic signal. Conspicuously, however, they comprise a cluster of phosphorylation sites.