A trypsin-like serine proteinase from plasmodial membrane of Physarum polycephalum

A trypsin-like serine proteinase was found in a plasma membrane fraction from the plasmodia of Physarum polycephalum, It was solubilized from the plasma membrane fraction with 1% (w/v) Triton X-100 and purified by a series of column chromatographic steps followed by polyacrylamide gel electrophoresis (PAGE). The molecular mass of the enzyme was estimated to be 130 kDa by SDS-PAGE under non-reducing conditions. This enzyme exhibited maximal activity at pH 9 and at 35°C with the substrate Boc-Phe-Ser-Arg-MCA, and was sensitive to various serine proteinase inhibitors and trypsin inhibitors. The enzyme was significantly labeled with [³H] diisopropylfluorophosphate (DFP); however, a clear decrease in the amount of the enzyme bound-[³H]DFP was observed in the presence of p-nitrophenyl-p'-guanidinobenzoate (NPGB), an active site titrant for trypsin. The activity was completely inhibited in the presence of EDTA or EGTA, but the metal-depleted enzyme was reactivated by the addition of Ca²⁺. Moreover, this enzyme was capable of cleaving several synthetic peptides at the carboxyl side of arginine, but did not work on synthetic substrates for chymotrypsin, elastase and aminopeptidase. The enzyme showed an affinity to Con A, indicating that it contains a carbohydrate moiety. Inhibition of the activity by a serine proteinase inhibitor resulted in cessation of the DNA synthesis in growing plasmodia without loss of the protein synthesis. Thus, this trypsin-like serine proteinase may have a regulatory role in the growth and development of the slime mould.