Characterization of a soluble M_r‐30000 catalytic fragment of the neuronal calmodulin‐dependent protein kinase II

Chymotryptic digestion of postsynaptic densities releases a soluble, catalytically active fragment of the α (M_r 50000) subunit of the neuronal cytoskeletal calmodulin‐dependent protein kinase II. The purified soluble form of the kinase likewise yields the fragment. Denaturation of the enzyme results in more extensive proteolytic degradation. ¹²⁵I‐Iodopeptide maps of the isolated catalytic portions of both forms of the enzyme are similar and are contained within the map of the isolated α subunit. Catalytic fragments of both forms of the enzyme comigrate on two‐dimensional SDS‐PAGE/isoelectric focusing with pI 6.7–7.2. The fragment phosphorylates microtubule‐associated protein (MAP‐2) but is not activated by Ca⁺²/calmodulin nor is it inhibited by trifluoperazine. K_m values for MAP‐2 and ATP are indistinguishable from those of the holoenzyme, while the V_max is similar to that of the holoenzyme activated with Ca⁺²/calmodulin. Overlays of Western blots of fragment with ¹²⁵I‐calmodulin shows a loss of calmodulin binding. Both the number of phosphorylation sites and the ability to autophosphorylate are markedly reduced in the catalytic fragment. Evaluation of the hydrodynamic parameters of the purified fragment yielded M_r value of 25600 with a frictional ratio (f/f_o) of 1.12; the M_r value determined by SDS‐PAGE was 30000. Thus, the catalytic fragment appears to represent an activated form of the kinase with a monomeric, globular structure unlike the native enzyme which exhibits oligomerization and cytoskeletal association. These results are consistent with a tertiary structure for the calmodulin‐dependent protein kinase that contains distinct domains responsible for catalytic activity, regulation by calmodulin, cytoskeletal association and the multimeric organization of enzyme subunits.