Sites of interaction of calmodulin with trifluoperazine and glucagon.

The combination of glucagon with calmodulin alters the microenvironment of Tyr-99, but not Tyr-138, and is blocked by the binding of trifluoperazine by calmodulin. Trp-25 of glucagon is probably involved in the zone of interaction, which may also overlap one or more strong binding sites for trifluoperazine. From energy transfer measurements, one strong binding site for trifluoperazine probably involves the N-terminal region of binding domain III. Energy transfer and other evidence suggest that the zone of contact with glucagon involves the N-terminal region of binding domain III.