Nuclear accumulation and homeostasis of the unusual polymer beta-poly (L-malate) in plasmodia of Physarum polycephalum.

The plasmodium of Physarum polycephalum specifically contains an unusual polyester, beta-poly(L-malate), which is not found in any of the mononucleate forms of its life cycle. Plasmodia growing on D-glucose have been analyzed for beta-poly(L-malate) in nuclei, cytosol and culture medium after cell fractionation, purification by chromatography on DEAE-cellulose and digestion of proteins/nucleic acids. Nuclei contained 400 micrograms polymer per 1 g of plasmodia, corresponding to a nuclear concentration of 230 mM L-malyl residues, not depending on growth rates, lengths of growth periods, and the growth form as micro- or macroplasmodia. The synthetic rate increased during the phase of rapid enlargement of the nuclei after mitosis. Beta-Poly(L-malate) was polydisperse in molecular mass, these tending to be higher in nuclear than in cytosolic extracts and being lowest in the culture medium. Beta-Poly(L-malate) was not degraded when contained in plasmodia, in contrast to degradation and the occurrence of low molecular mass polymer in the culture medium. During pulse-chase feeding with D-[14C]glucose (0.8 micrometerCi/mol), beta-[14C]poly(L-malate) appearance followed kinetics indicating a release of polymer from nuclei into the culture medium when it was in excess of a threshold. Injection experiments with purified beta-[14C]poly(L-malate) revealed a re-entry from the cytoplasm into the nuclei and thus the possibility of commutation between the cytoplasm and the nuclei. The observed homeostasis in nuclei supports the assumption that beta-poly(L-malate) plays an essential role in growing plasmodia.