In a previous paper (Muszbek et al. FEBS Letters 1977, 80, 308) the existence of a troponin C-like protein has been revealed in platelets. This protein was isolated, characterized and identified as the multifunctional Cadependent regulator protein (modulator). Here we show that there is a platelet protein which in 6·3 M urea if Ca2+ present can form a complex with modulator protein and with skeletal muscle troponin C, too. It withstands aceton treatment and can be extracted from platelet aceton

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... powder by 1 M KCl. Further purification could be achieved by affinity chromatography on an Affi-Gel 10-troponin C column. Modulator binding protein also copurified with platelet actomyosin though there was no detectable amount of modulator protein in this preparation. Since modulator protein appears to be responsible for the Ca2+ regulation of purified platelet myosin light chain kinase (Dabrowska and Harsthorne BBRC 1978, 85, 1352) the low Ca2+ sensitivity of platelet actomyosin may be due to the virtual absence of modulator and/or to the presence of modula tor binding prote in in thrombosthenin.