FUNCTIONAL-HETEROGENEITY OF ALTERNATIVELY SPLICED ISOFORMS OF DROSOPHILA CA2+ CALMODULIN-DEPENDENT PROTEIN-KINASE-II/

The gene for Drosophila calcium/calmodul-independent protein kinase II is alternatively spliced to generate up to 18 different proteins that vary only in a region analogous to the point where mammalian alpha, b eta, gamma, and delta isozymes show the greatest divergence from each other. To investigate the function of this variable region, we have ch aracterized the catalytic and structural properties of six of the Dros ophila isoforms. By several criteria (domain organization, low affinit y for calmodulin, holoenzyme structure, and ability to autophosphoryla te and become independent of calcium), these proteins are functional h omologues of the mammalian calcium/calmodulin-dependent protein kinase II. Two major isoform-specific catalytic differences were observed. F irst, the R3A isoform was found to have a significantly higher K-act f or calmodulin than the other isoforms. This indicates that the variabl e region, which is located distal to the calmodulin-binding domain, ma y play a role in activation of the enzyme by calmodulin. Decreased sen sitivity to calmodulin may be biologically important if free calmoduli n is limiting within the neuron. The second catalytic difference noted was that the R6 isoform had a significantly lower K-m for the peptide substrate used in this study. Although the variable region is not in the catalytic part of the enzyme, it may have an indirect function in substrate selectivity.