Cellular and molecular analysis of plasmodium development in Physarum

Physarum amoebae express a distinct fragmin-like actin-binding protein that controls in vitro phosphorylation of actin by the actin-fragmin kinase

Amoebae and plasmodia constitute the two vegetative growth phases of the Myxomycete Physarum. In vitro and in vivo phosphorylation of actin in plasmodia is tightly controlled by fragmin P, a plasmodium-specific actin-binding protein that enables actin phosphorylation by the actin-fragmin kinase. We investigated whether amoebal actin is phosphorylated by this kinase, in spite of the lack of fragmin P. Strong actin phosphorylation was detected only following addition of recombinant actin-fragmin kinase to cell-free extracts of amoebae, suggesting that amoebae contain a protein with properties similar to plasmodial fragmin. We purified the complex between actin and this protein to homogeneity. Using an antibody that specifically recognizes phosphorylated actin, we demonstrate that Thr203 in actin can be phosphorylated in this complex. A full-length amoebal fragmin cDNA was cloned and the deduced amino acid sequence shows 65% identity with plasmodial fragmin. However, the fragmins are encoded by different genes. Northern blots using RNA from a developing Physarum strain demonstrate that this fragmin isoform (fragmin A) is not expressed in plasmodia. In situ localization showed that fragmin A is present mainly underneath the plasma membrane. Our results indicate that Physarum amoebae express a fragmin P-like isoform which shares the property of binding actin and converting the latter into a substrate for the actin-fragmin kinase.

Molecular cloning, over-expression, developmental regulation and immunolocalization of fragminP, a gelsolin-related actin-binding protein from Physarum polycephalum plasmodia

FragminP is a Ca2+-dependent actin-binding and microfilament regulatory protein of the gelsolin family. We screened a Physarum polycephalum cDNA library with polyclonal fragminP antibodies and isolated a cDNA clone of 1,104 bp encoding 368 amino acids of fragminP, revealing two consensus phosphatidylinositol 4,5 bisphosphate-binding motifs in the central part of the protein. The first methionine is modified by an acetyl group, and three amino acids were missing from the protein coded for by the cDNA clone. Full-length recombinant fragminP was generated by PCR, purified after over-expression from Escherichia coli and displayed identical properties to native Physarum fragminP. Northern blot analysis against RNA, isolated from cultures at various stages of development, indicated that fragminP is absent from amoebae and that expression is initiated at an early stage during apogamic development, in a similar way to that observed for the profilin genes. In situ immunolocalization of fragminP in Physarum microplasmodia revealed that the protein is localized predominantly at the plasma membrane, suggesting a role in the regulation of the subcortical actin meshwork. Our data indicate that we have isolated the plasmodium-specific fragminP cDNA (frgP) and suggest that, in each of its two vegetative cell types, P. polycephalum uses a different fragmin isoform that performs different functions.