DESCRIPTION: Several hnRNP proteins cycle between the nucleus and cytoplasm, exiting the nucleus associated with mRNA and re-entering by a unique mechanism that is dependent on continued transcription by RNA polymerase II. This suggests that regulatory signals are transduced between the nucleus and cytoplasm. Preliminary fractionation of cell extracts used to support protein import in permeabilized cells indicates that the factors necessary for the import of classical NLS-containing proteins can be separated from those required for hnRNP A1 protein. This proposal aims to purify and characterize the factors required for nuclear import of hnRNP A1. Their regulation by RNA polymerase II transcription will be addressed. Both permeabilized cells and microinjection into Xenopus oocytes will be used to identify relevant factors.