Human calcium-calmodulin dependent protein kinase I: cDNA cloning, domain structure and activation by phosphorylation at threonine-177 by calcium-calmodulin dependent protein kinase I kinase

Human Ca²⁺-calmodulin (CaM) dependent protein kinase I(CaMKI) encodes a 370 amino acid protein with a calculated M, of 41 337. The 1.5 kb CaMKI mRNA is expressed in many different human tissues and is the product of a single gene located on human chromosome 3. CaMKI 1-306, was unable to bind Ca²⁺-CaM and was completely inactive thereby defining an essential component of the CaM-binding domain to residues C-terminal to 306. CaMKI 1-294 did not bind CaM but was fully active in the absence of Ca²⁺-CaM, indicating that residues 295-306 are sufficient to maintain CaMKI in an auto-inhibited state. CaMKI was phosphorylated on Thr177 and its activity enhanced ~25-fold by CaMKI kinase in a Ca²⁺-CaM dependent manner. Replacement of Thr177 with Ala or Asp prevented both phosphorylation and activation by CaMKI kinase and the latter replacement also led to partial activation in the absence of CaMKI kinase. Whereas CaMKI 1-306 was unresponsive to CaMKI kinase, the 1-294 mutant was phosphorylated and activated by CaMKI kinase in both the presence and absence of Ca²⁺-CaM although at a faster rate in its presence. These results indicate that the auto-inhibitory domain in CaMKI gates, in a Ca²⁺-CaM dependent fashion, accessibility of both substrates to the substrate binding cleft and CaMKI kinase to Thr177. Additionally, CaMKI kinase responds directly to Ca²⁺-CaM with increased activity.