Characterization of cis-acting signals for nuclear import and retention of the La (SS-B) autoantigen.

The La (SS-B) autoantigen is a 47-kDa protein which binds to the 3' termini of nascent RNA polymerase III transcripts and to a number of viral leader RNAs. The La protein plays a direct role in the termination of RNA polymerase III transcription and recent findings have suggested an additional role in several aspects of translation of (viral) mRNAs. In this study we have addressed the intracellular trafficking of the La protein and characterized cis-acting elements involved in nuclear import and retention in Xenopus laevis oocytes by microinjection of in vitro translated La protein. The steady-state distribution of recombinant human La protein was, like the endogenous Xenopus La protein, mainly nuclear. Nuclear import of La appeared to be energy-dependent and is governed by a nuclear localization signal (NLS) located in the extreme C-terminal part of the protein, resembling the consensus bipartite NLS. Another sequence element in La, which completely corresponds to the bipartite NLS consensus, appeared to be nonfunctional in nuclear import of the La protein. Nuclear accumulation of La was found to be mediated by retention in the nuclear compartment. The N-terminal RNA binding domain of La is not involved in this retention, but sequence elements in the central region of the polypeptide (amino acids 165 to 337) appear to be required. Amino acids 266-269 as well as 313-337 were found to be of major importance for retention in the nucleus.