Casein kinases I and II from squid brain exhibit selective neurofilament phosphorylation

In studies of the function of neurofilaments in the squid giant axon we showed that isolated neurofilament preparations from axoplasm are associated with high levels of casein kinase-like activity. To determine the role of these kinases in phosphorylation of neurofilament proteins, we isolated two kinases from squid brain which are also found in axoplasm, CK I and CK II. The CKI is similar to this axonal neurofilament-associated CKI-like kinase activity. CK I displayed a high specificity for the squid high molecular weight (NF220) and rat high molecular weight (NF-H) neurofilament proteins relative to alpha-casein, phosvitin, and middle (NF-M) and low (NF-L) rat neurofilament proteins. The brain CKII, with activity similar to that found in axoplasm, but not associated with neurofilaments, poorly phosphorylated NF220 and NF-H, while demonstrating similar affinities, relative to CK I, for NF-M, NF-L, alpha-casein, and phosvitin. The high affinity of squid neuronal CKI for squid NF220 and rat NF-H and its association with axonal neurofilaments suggest that this kinase may have a specific role in neurofilament phosphorylation essential for interaction with other cytoskeletal elements in the axon.