IDENTIFICATION OF FUNCTIONING REGULATORY SITES AND A NEW MYOSIN BINDING-SITE IN THE C-TERMINAL 288 AMINO-ACIDS OF CALDESMON EXPRESSED FROM A HUMAN CLONE

A partial clone of caldesmon, coding for the C-terminal 288 amino acids, was isolated from a human fetal liver cDNA library and sequenced. Expression of the clone in Escherichia coli produced a peptide called H1 (M(r) 32 549), which inhibited tropomyosin-enhanced actomyosin Mg2+-ATPase activity by 90% with half maximal inhibition at 0.03-0.04 mol H1 per mol actin. The inhibition could be reversed by Ca2+-calmodulin. H1 bound actin, Ca2+-calmodulin and tropomyosin and smooth muscle myosin with high affinities. This latter finding shows the presence of a second myosin-binding site in caldesmon. This was confirmed in thrombic digests of native sheep aorta and chicken gizzard caldesmon.