A DEAD-BOX-FAMILY PROTEIN IS REQUIRED FOR NUCLEOCYTOPLASMIC TRANSPORTOF YEAST MESSENGER-RNA

An enormous variety of primary and secondary mRNA structures are compa tible with export from the nucleus to the cytoplasm. Therefore, there seems to be a mechanism for RNA export which is independent of sequenc e recognition. There nevertheless is likely to be some relatively unif orm mechanism which allows transcripts to be packaged as ribonucleopro tein particles, to gain access to the periphery of the nucleus and ult imately to translocate across nuclear pores. To study these events, we and others have generated temperature-sensitive recessive mRNA transp ort (mtr) mutants of Saccharomyces cerevisiae which accumulate poly(A) (+) RNA in the nucleus at 37 degrees C. Several of the corresponding g enes have been cloned. Upon depletion of one of these proteins, Mtr4p, conspicuous amounts of nuclear poly(A)(+) RNA accumulate in associati on with the nucleolus. Corresponding dense material is also seen by el ectron microscopy, MTR4 is essential for growth and encodes a novel nu clear protein with a size of similar to 120 kDa. Mtr4p shares characte ristic motifs with DEAD-box RNA helicases and associates with RNA. It therefore may well affect RNA conformation. It shows extensive homolog y to a human predicted gene product and the yeast antiviral protein Sk i2p. Critical residues of Mtr4p, including the mtr4-1 point mutation, have been identified. Mtr4p may serve as a chaperone which translocate s or normalizes the structure of mRNAs in preparation for export.