[Use of gene fusion in the study of the interaction of ribosomal protein L45 with the Saccharomyces cerevisiae ribosome].

Cytoplasmic ribosomes from all cells have a set of highly acidic proteins (proteins A; pI less than 4.0). Proteins A are indispensable for ribosomal function since they mediate the interaction between GTP dependent translational factors and the ribosome. From gene cloning and DNA sequence analysis, during the past few years the primary structure of several prokaryotic and eukaryotic proteins A has been deduced. When their primary structure is compared, it is evident that these proteins have three well defined domains: firstly the aminus terminal domain, comprising the first 40 to 50 amino acids; secondly, the central domain, rich in non-polar amino acids and a high percent of alpha helix; and thirdly, the alpha carboxyl terminal domain, relatively acidic and highly conserved throughout evolution. In Saccharomyces cerevisiae the four genes coding for proteins A L45, L44, L44' and A1 have been cloned and sequenced. This enabled us to study the physiological role of each of the three domains in proteins A. Plasmid YEp45-1 encompassing a 596 base pairs stretch of the L45 genomic DNA fused to the LacZ gene was constructed and used to transform yeast cells. The chimeric gene expressed an hybrid protein in which the covalently linked beta-galactosidase enzyme follows after the first 75 amino acids from the alpha amino terminal domain of protein A L45. After cell fractionation of yeast transformants, beta-galactosidase activity was found specifically bound to the 80S ribosome and polysome particles. From these results we conclude that the amino terminal domain of protein L45 interacts to the ribosome. We postulate, by inference, that the carboxyl terminal domain of proteins A interacts with the GTP dependent cytoplasmic translational factors.