Caldesmon, a protein originally found in chicken gizzard, was concluded also to be present in bovine aorta, uterus, and human platelets by demonstration of a protein with the following properties: (a) Ca2+-dependent calmodulin-binding; (b) binding to F actin in such way that the binding was broken on Ca2+-dependent binding of calmodulin; (c) cross-reactivity in immune blotting procedures with affinity-purified antibody against gizzard caldesmon; (d) similar subunit M r-values on SDS-gel to those of gizzard caldesmon. Like gizzard caldesmon, platelet caldesmon was composed of two polypeptide bands of M r 150000 and 147000, but caldesmon in aorta and uterus gave a single band of M r 150000. A polypeptide of M r 165000 that was immunologically distinct from caldesmon but, like caldesmon, bound to calmodulin and F actin in a flip-flop fashion, was also demonstrated in aorta and uterus.
