The importance of importin

The accumulation of karyophilic proteins in the nucleus requires cytoplasmic factors. Cell-free systems that reconstitute nuclear protein import have been used to identify several of these factors and to define the biochemical requirements for the import process. Recently, one factor has been purified and cloned from Xenopus and identified as a homologue of the 'suppressor of RNA polymerase l' (SRP1) gene originally described in yeast. This factor belongs to a closely related group of proteins that may share similar functions in nuclear protein transport.