INTERACTION OF ISOFORMS OF S100 PROTEIN WITH SMOOTH-MUSCLE CALDESMON

Interaction of S100a and S100b with duck gizzard caldesmon was investi gated by means of native gel electrophoresis, fluorescent spectroscopy and disulfide crosslinking. Both isoforms of S100 interact with intac t caldesmon and its C-terminal deletion mutant 606C (residues 606-756) with apparent K-d of 0.2-0.6 mu M thus indicating that the S100-bindi ng site is located in the C-terminal domain of caldesmon. The single S H group of duck gizzard caldesmon can be crosslinked to Cys-84 of the beta-chain or to Cys-85 of the alpha-chain of S100. Crosslinking of S1 00 reduces the inhibitory action of caldesmon on actomyosin ATPase act ivity. S100 reverses the inhibitory action of intact caldesmon and its deletion mutants 606C (residues 606-756) and H9 (residues 669-737) as effectively as calmodulin. S100a has higher affinity to caldesmon and is more effective than S100b in reversing caldesmon-induced inhibitio n of actomyosin ATPase activity. Although monomeric (calmodulin, tropo nin C) and dimeric (S100) Ca-binding proteins have different sizes and structures they interact with caldesmon in a very similar fashion. (C ) 1998 Federation of European Biochemical Societies.