HETEROLOGOUS EXPRESSION OF THE HIGHLY CONSERVED ACIDIC RIBOSOMAL PHOSPHOPROTEINS FROM DICTYOSTELIUMM DISCOIDEUM IN SACCHAROMYCES-CEREVISIAE

The genes encoding the acidic ribosomal phosphoproteins DdP1 and DdP2 from Dictyostelium discoideum have been cloned into yeast plasmid vect ors under the control of the inducible GAL1 promoter. These constructi ons have been used to transform S. cerevisiae strains D45 and D67 lack ing the equivalent ribosomal components. The D. discoideum genes are p roperly transcribed when cells are grown in the presence of the induce r galactose and the mRNAs incorporated into polysomes. However, the he terologous ribosomal proteins are not able to rescue the growth defici ency in S. cerevisiae caused by the absence of their own ribosomal pro teins. When the heterologous proteins are analyzed using specific anti bodies, only protein DdP1 is found in the ribosomes of the transformed S. cerevisiae D67 strain. No other heterologous protein is found in a ny other transformed strain, suggesting that the heterologous acidic r ibosomal components are rapidly degraded when they are not bound to th e ribosomes. The results indicate that D. discoideum DdP1 protein is a ble to interact with the yeast ribosome, though the interaction is fun ctionally inefficient. Protein DdP2, in spite of having a higher seque nce similarity to its yeast counterparts, is completely inactive in S. cerevisiae. Since the P proteins from both organisms have extensive a mino acid sequence similarity ranging from 60% to 70%, these results w arns about establishing a direct relationship between the extent of am ino acid sequence similarity and the capacity of heterologous proteins to be functional in host species. Moreover, our data suggest that evo lution affected the interaction of the acidic proteins with the riboso me rather than the structural features responsible for their primary f unctions.