Binding of simple peptides, hormones, and neurotransmitters by calmodulin

Publisher Website
Google Scholar
Abstract
A fluorescent conjugate of porcine calmodulin was prepared with 5-(dimethylamino)-1-naphthalene-sulfonyl chloride that is highly sensitive to both Ca binding and protein binding. The fluorescence of this conjugate in addition to the intrinsic peptide fluorescence was used to show that ACTH, .beta.-endorphin, glucagon, and substance P undergo Ca-dependent binding by calmodulin, with competition for common binding sites. The Kd determined in the presence of 0.85 mM CaCl2 and 0.2 N KCl, pH 7.3 at 25.degree. C, range from 1.5 .mu.M to 3.4 .mu.M. .alpha.-MSH, bombesin and somatostatin also bind, with Kd between 60 .mu.M and 90 .mu.M. Angiotensins I and III, bradykinin, neurotensin, physaelemin, substance P octapeptide, insulin, and Leu- and Met-enkephalin show little or no binding. Sequence comparisons show that the peptides that bind calmodulin well contain regions structurally similar to the recognition sequence for the cAMP-dependent protein kinase and to the sequences surrounding phosphorylated serine residues in several calmodulin binding proteins. Modification of calmodulin binding sites in calmodulin-dependent proteins is one of the functions of protein kinase. Ca has a dual role in peptide binding by calmodulin. The occupation of Ca binding sites having a pK .apprx. 4 results in a 2-fold increase in peptide binding affinity.
Keywords