The Domain Structure of the Calmodulin-Dependent Phosphodiesterase Isozymes
The calmodulin-dependent cyclic nucleotide phophodiesterases (CaM-PDEs) hydrolyze both cAMP and cGMP and are stimulated 5–15 fold by the binding of calcium and calmodulin (CaM)1,2. These isozymes provide a mechanism whereby calcium, acting as a second messenger, can alter the intracellular levels of cAMP and cGMP. The CaM-PDEs are members of a large family of PDE isozymes that differ in molecular weight, substrate specificity, kinetic parameters and most significantly in mode of regulatory control. Protein and cDNA sequence studies show that PDEs from diverse species (Drosophila, Yeast, and Bovine) contain a 270 amino acid segment displaying significant sequence similarities. We have postulated3,4 that this homologous domain comprises the catalytic site of these isozymes, while the non-homologous regions have domains that are involved in regulatory function (see Figure 1). Evidence that the conserved domain is associated with the catalytic site is derived from limited proteolysis of the 61K CaM-PDE which produces a 3 6K fragment that is fully active, yet CaM-independent. This 36K fragment, which is produced by trypsin treatment in a method similar to Kincaid5, can be immunoblotted by anti-peptide antisera directed against sequence from the conserved domain4.
KeywordsLimited Proteolysis Cyclic Nucleotide Phosphodiesterase Amino Acid Segment Muscle Myosin Light Chain Kinase Atrial Strip
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