NUCLEOCYTOPLASMIC DISTRIBUTION OF HUMAN HNRNP PROTEINS - A SEARCH FORTHE TARGETING DOMAINS IN HNRNP A1

hnRNP Al (34 kDa) is an RNA binding protein consisting of two tandemly arranged RNA binding domains C-terminally linked to a glycine-rich au xiliary domain (2xRBD-Gly), Al belongs to the set of polypeptides that bind nascent hnRNA in the nucleus to form the so called hnRNP complex es. These complexes seem to be involved both in pre-mRNA processing an d in the nuclear export of mRNA. In fact Al, along with other hnRNP pr oteins, is exported from the nucleus probably bound to mRNA and is imm ediately re-imported. Al nuclear re-import, which requires active tran scription, is not mediated by a canonical nuclear localisation signal (NLS). To identify the determinants of Al subcellular localisation ne de,eloped an expression vector for studying the localisation, in trans iently transfected cells, of the different structural motifs of Al fus ed to a small reporter protein (chloramphenicol acetyltransferase, CAT ; 26 kDa), We demonstrate that a 30 amino acid sequence in the glycine -rich domain (YNDFGNYNNQSSNFGPMKGGNFGGRSSGPY), WHICH bears no resembla nce to canonical NLS, is necessary and sufficient to target the protei n to the nucleus, Our data suggest that this targeting sequence might act by mediating the interaction of Al with a NLS-containing nuclear i mport complex, On the other hand, the nuclear export of Al requires at least one RNA binding domain in accord with the hypothesis that iii e xits from the nucleus bound to mRNA. We propose a mechanism for the nu cleo-cytoplasmic shuttling of Al that envisages a specific role for th e different structural domains and can explain the dependence of nucle ar import from active transcription.