Recently published data [Vorotnikov & Gusev (1990) FEBS Lett. 277, 134-136] indicate that smooth muscle caldesmon interacts with a mixture of soybean phospholipids (azolectin). Continuing this investigation, we found that duck gizzard caldesmon interacts more tightly with acidic (phosphatidylserine) than with neutral (phosphatidylcholine) phospholipids. A high concentration of Ca2+ (50 microM) decreased the interaction of caldesmon with phosphatidylserine. Among chymotryptic peptides of caldesmon, only those having molecular masses of 45, 40, 23, 22 and 20 kDa were able to specifically interact with phospholipids. These peptides, derived from the C-terminal part of caldesmon, contained the sites phosphorylated by Ca2+/phospholipid-dependent protein kinase, and phosphorylation catalysed by this enzyme decreased the affinity of these peptides for phospholipids. In the presence of Ca2+, calmodulin competed with phospholipids for the interaction with the caldesmon peptides. The C-terminal part of caldesmon contains three peptides with a primary structure similar to that of the calmodulin- and phospholipid-binding site of neuromodulin. These sites may be involved in the interaction of caldesmon with calmodulin and phospholipids.

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