Spinach calmodulin: isolation, characterization, and comparison with vertebrate calmodulins

Abstract

Calmodulin is the name proposed for a multifunctional, Ca binding protein whose presence was detected in a number of eukaryotic cells. Calmodulin was isolated from spinach leaves (S. oleracea), characterized and compared to vertebrate calmodulins. Quantitative recovery data for a rapid-isolation protocol demonstrate that calmodulin is a major constituent of spinach leaves. Spinach calmodulin is indistinguishable from vertebrate calmodulins in phosphodiesterase activator activity using vertebrate brain phosphodiesterase and in quantitative immunoreactivity using antiserum made against vertebrate calmodulin. Spinach calmodulin is readily distinguished from vertebrate and invertebrate calmodulins in electrophoretic mobility and in amino acid composition. Spinach calmodulin, like vertebrate calmodulins, lacks tryptophan and contains 1 mol each of N.epsilon.-trimethyllysine and histidine/17,000 g of protein. In contrast to vertebrate calmodulins, spinach calmodulin has only 1 tyrosinyl residue and has a threonine/serine ratio of 1.3. While amino acid compositions indicate differences between spinach and vertebrate calmodulins, isolation and characterization of tryptic peptides containing the single histidinyl and N.epsilon.-trimethyllysyl residues and both prolinyl residues indicate that these regions in spinach calmodulin are similar to the corresponding regions in vertebrate calmodulin. These studies more fully define the general and specific characteristics of calmodulins and indicate that calmodulin structure is not as highly conserved among all eukaryotes as it is among vertebrates and invertebrates.