Molecular Identification and Characterization of a Family of Kinases with Homology to Ca2+/Calmodulin-dependent Protein Kinases I/IV

Abstract

Despite the critical importance of Ca 2؉ /calmodulin (CaM)-dependent protein kinase (CaMK) II signaling in neuroplasticity, only a limited amount of work has so far been available regarding the presence and significance of another predominant CaMK subfamily, the CaMKI/CaMKIV family, in the central nervous system. We here searched for kinases with a core catalytic structure similar to CaMKI and CaMKIV. We isolated full-length cDNAs encoding three mouse CaMKI/CaMKIV-related kinases, CLICK-I (CL1)/doublecortin and CaM kinaseLike (DCAMKL)1, CLICK-II (CL2)/DCAMKL2, and CLICK-I,II-related (CLr)/DCAMKL3, the kinase domains of which had an intermediate homology not only to CaMKI/CaMKIV but also to CaMKII. Furthermore, CL1, CL2, and CLr were highly expressed in the central nervous system, in a neuron-specific fashion. CL1␣ and CL1␤ were shorter isoforms of DCAMKL1, which lacked the doublecortin-like domain (Dx). In contrast, CL2␣ and CL2␤ contained a full N-terminal Dx, whereas CLr only possessed a partial and dysfunctional Dx. Interestingly, despite a large similarity in the kinase domain, CL1/CL2/CLr had an impact on CRE-dependent gene expression distinct from that of the related CaMKI/CaMKIV and CaMKII. Although these were previously shown to activate Ca 2؉ /cAMP-response element-binding protein (CREB)-dependent transcription, we here show that CL1 and CL2 were unable to significantly phosphorylate CREB Ser-133 and rather inhibited CRE-dependent gene expression by a dominant mechanism that bypassed CREB and was mediated by phosphorylated TORC2.The availability of the human and other mammalian genome sequences provides a powerful means to dissect en masse the function of a particular class of proteins whose primary function can be deduced based on their primary sequences. The entire catalog of putative mammalian protein kinases, or kinome, has recently been established and curated (1, 2). This achievement per se is significant and now affords a better understanding of the biological role of kinases as regulatable switches under many physiologically and pathophysiologically critical circumstances. However, precise knowledge about the molecular characteristics of individual kinase molecules is still lacking (3).As part of the large list of serine/threonine and tyrosine kinase families, the Ca 2ϩ /calmodulin (CaM) 3 -dependent protein kinase (CaMK) group stands out by the large number of its constituent kinases (1-3). Despite its nomenclature, however, only the classic CaMK subgroups such as CaMKII family, or the CaMKI/CaMKIV family, are genuinely catalytically Ca 2ϩ /CaMdependent. Most of the kinases of the CaMK group actually lack the characteristic Ca 2ϩ /CaM-sensitive regulatory domain. They nonetheless belong to the CaMK group, because they share in common a significant homology in the primary structure of their kinase domains. Several classic CaMKs such as CaMKII and CaMKIV are highly expressed in the central nervous system and have been convincingly shown to play a critical role in long term synaptic plasticity and...

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