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A group of prokaryotic actin-related proteins (PARP) with an M r of 43000 was detected in Actinobacillus pleuropneumoniae. These proteins were enriched by a depolymerization/polymerization cycle, under similar conditions to those used to polymerize muscle actin, and purified by affinity chromatography on a DNase I-Sepharose column. Three isoforms of A. pleuropneumoniae PARP (Ap-PARP) with pI values of 5·8, 6·15 and 6·2 were detected. Ap-PARP were recognized by four different anti-actin antibodies (one anti-muscle and three anti-cytoplasmic isoforms). Ap-PARP were also recognized by antibodies against Anabaena variabilis PARP (Av-PARP) and against actin-binding proteins such as α-actinin and spectrin, and also by a monoclonal antibody against heat-shock cognate protein 70 (Hsc70). Specific binding of phalloidin to Ap-PARP was detected both in permeabilized cells and in vitro. Purified Ap-PARP can polymerize under similar conditions to those required for skeletal muscle actin polymerization and the filaments formed appear to be decorated with myosin subfragment-1 (S1) as observed by transmission electron microscopy. The amino acid composition of Ap-PARP revealed more similarities to muscle γ-actin and the cytoplasmic β-actin isoform than to eukaryotic actin-related proteins.
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