Properties of two isoforms of human blood platelet α‐actinin

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Abstract
The structural and functional properties of the aa (2 × 97 kDa) and cc (2 × 94 kDa) isoforms of platelet α‐actinin have been compared. Structural differences between aa and cc are revealed by their peptide maps, obtained from limited proteolysis, and by their immunological cross‐reactivity. Both isoforms stimulate the Mg ATPase activity of actomyosin, bind to F‐actin (high‐speed sedimentation) and cross‐link or gel actin filaments (low‐speed sedimentation and viscometry), in a calcium‐dependent manner. The study of the interaction with F‐actin indicates that the binding of 1 molecule of aa or ccα‐actinin/9‐11 actin monomers is sufficient to produce maximal gelation in the presence of EGTA. CaCl2 at 0.1 mM strongly inhibits the binding of aa to F‐actin and weakly that of cc, while it inhibits similarly the cross‐linking of either aa or cc. The cross‐linking efficiency of cc is 9, 7, 1.7 and 1.3 times higher than that of aa at 4, 20, 30 and 37°C, respectively. The bb form (2 × 96 kDa), which is a proteolytic product of aa [Y. Gache et al. (1984) Biochem. Biophys. Res. Commun. 124, 877‐881], behaves roughly as aa, but the calcium sensitivity of its binding to F‐actin is intermediate between that of aa and cc. These results suggest that the effect of Ca2+ concentration on the binding of platelet α‐actinin to F‐actin may be partly dissociated from the effect on the cross‐linking.