The rapid intranuclear accumulation of preexisting proteins in response to high plasmodial density in Physarum polycephalum

When actively growing microplasmodia of the lower eukaryote Physarum polycephalum are gently pelleted and allowed to stand at high plasmodial densities for 45 min, three specific nuclear acidic proteins undergo dramatic quantitative changes. Two major proteins of molecular weight 46 000 and 94 000 increase 110 and 320%, respectively. The increase in these two proteins is not markedly attenuated during periods when 88% total protein synthesis is blocked by cycloheximide, and the specific radioactivities of these proteins from prelabeled and continuously labeled control and pelleted plasmodia are essentially identical. A third protein of molecular weight 34 000 decreases by 51 % during the 45 min period and when cycloheximide is present, a 36% decrease in this protein still occurs. The rapid changes which occur in these three proteins in response to high plasmodial density also develop, together with many other changes, during plasmodial differentiation, but only after about 6 h of starvation. It is concluded that the rapid increase in the 46 000 and 94 000 mol. wt proteins results from protein transfer phenomena rather than de novo synthesis and that these proteins perhaps function in the early reorganization of cell metabolism rather than in structural differentiation. In further comparative studies it has been observed that mature spherules of P. polycephalum contain a major acidic protein not present in growing or differentiating plasmodia and also that the complement of residual acidic proteins differs in starvation-induced vs cold-induced spherules.