Interaction of a protein-bound polysaccharide (PSK) with chicken gizzard caldesmon

The interaction of a protein-bound polysaccharide (PSK) with caldesmon was studied in detail using sedimentation, low-shear viscometry, electron microscopy and affinity chromatography. The binding of caldesmon to F-actin was concentration-dependently inhibited by PSK. Bundle formation of actin filaments owing to caldesmon was also inhibited by PSK. Caldesmon bound to a PSK-Sepharose 4B column at low ionic strength was released at about 400 mM NaCl, whereas G-actin was not retained by the column. Treatment of caldesmon with chymotrypsin produced major fragments near 100, 80, 60, 38 and 25 kDa. In contrast, 60 and 25 kDa fragments were rarely formed by this treatment in the presence of PSK. Fragments of 80 and 38 kDa, major products produced by chymotrypsin, bound individually to a PSK-Sepharose column, indicating that caldesmon has at least two binding sites for PSK. Addition of calcium and calmodulin partially released caldesmon from actin filaments. PSK-dependent release of caldesmon was also observed in the presence of calcium and calmodulin.