Increased rates of decay and reduced levels of accumulation of the major poly(A)-associated proteins of Dictyostelium during heat shock and development

Abstract

Two major polypeptide species, 31,000 Mr (p31) and 31,500 Mr (p31.5), are associated with the 3' poly(A) tails of Dictyostelium mRNAs. We have measured the accumulation of newly synthesized p31 and p31.5 and the decay of preexisting p31 and p31.5 during heat shock and early development. Only trace amounts of newly synthesized p31 and p31.5 accumulate at elevated temperatures, indicating that these polypeptides are not heat shock proteins. In addition, preexisting p31 and p31.5 are rapidly degraded in heat-shocked cells. This degradation is selective and occurs simultaneously with a sharp drop in the rate of translational initiation. Similarly, in early development, a time when the rate of translational initiation is also sharply reduced, only trace amounts of newly synthesized p31 and p31.5 accumulate and most of the preexisting p31 and p31.5 is rapidly degraded. When translational elongation is inhibited with cycloheximide, preexisting p31 and p31.5 remain stable. Therefore, a correlation seems to exist between the abundance and stability of these poly(A)-associated proteins and the rate of translational initiation. Our results are consistent with the proposed role of the poly(A)-protein complex in translation and do not support the findings of Schönfelder et al. [Schönfelder, M., Horsch, A. & Schmid, H.-P. (1985) Proc. Natl. Acad. Sci. USA 82, 6884-6888] that the 73,000 Mr HeLa cell poly(A)-binding protein and the major 73,000 Mr mammalian heat shock protein (i.e., hsp70) are identical.