IN-VIVO EVIDENCE FOR INVOLVEMENT OF A 58 KDA COMPONENT OF NUCLEAR PORE-TARGETING COMPLEX IN NUCLEAR-PROTEIN IMPORT

We recently showed that a nuclear location signal (NLS)-containing kar yophile forms a stable complex with cytoplasmic components for nuclear pore-targeting. The complex, termed nuclear pore-targeting complex (P TAC), contained two essential proteins of 54 and 90 kDa, respectively, as estimated by electrophoresis. In this study, we found that the 54 kDa component of PTAC is the mouse homologue of Xenopus importin (m-im portin). Cytoplasmic injection of the antibodies raised against recomb inant m-importin showed an inhibitory effect on nuclear import of a ka ryophile in living mammalian cells. A portion of cytoplasmically injec ted antibodies migrated rapidly into the nucleus, indicating dynamic m ovement of this protein across the nuclear envelope. Moreover, the inj ected antibodies co-precipitated the karyophile, in an NLS-dependent m anner, with endogenous m-importin in the cytoplasm. These results prov ide in vivo evidence that m-importin is involved in nuclear protein im port through association with a NLS in the cytoplasm before nuclear po re binding.