Analysis of the uncharacterized La-related protein, Slr1p from Schizosaccharomyces pombe

La is a ubiquitous RNA-binding phosphoprotein found in nearly all eukaryotes. La binds to nascent polymerase III transcripts via their UUU-3'OH motif and protects these RNAs from 3'exonucleolytic digestion. La also exhibits RNA chaperone activity, is involved in intracellular trafficking, and stimulates the translation of certain subsets of mRNA via unknown mechanisms. Structurally, all La proteins harbour an N-terminal La module consisting of a La motif (LAM) and an RNA recognition motif. The LAM displays a high degree of conservation, not only among genuine La proteins but also in La-related proteins (LARPs) of which 4 families have been identified: LARP 1, 4, 6 and 7. Although they fulfill important cellular functions, LARPs are generally poorly understood. In an effort to characterize LARPs in greater detail, a simple LARP from Schizosaccharomyces pombe, Slrlp, was chosen as an ideal candidate to study. From an evolutionary perspective, we propose that Slr1p constitutes an ancestor to LARPs in higher eukaryotes, and therefore, characterization of its function may provide insight into the divergent roles of its extended relatives. We performed RNA chaperone assays, electromobility shift assays, localization studies via indirect immunofluorescence, as well as co-immunoprecipitation and mass spectrometry to determine Slr1p associated factors. Taken together, our results suggest that Slr1p is an RNA chaperone, it associates with fatty acid synthase as well as translation initiation factors in the cytoplasm, and that it binds to RNAs in a UUU-3'OH independent manner.