CASK Functions as a Mg2+-Independent Neurexin Kinase

Abstract

CASK is a unique MAGUK protein that contains an N-terminal CaM-kinase domain besides the typical MAGUK domains. The CASK CaM-kinase domain is presumed to be a catalytically inactive pseudokinase because it lacks the canonical DFG motif required for Mg2+ binding that is thought to be indispensable for kinase activity. Here we show, however, that CASK functions as an active protein kinase even without Mg2+ binding. High-resolution crystal structures reveal that the CASK CaM-kinase domain adopts a constitutively active conformation that binds ATP and catalyzes phosphotransfer without Mg2+. The CASK CaM-kinase domain phosphorylates itself and at least one physiological interactor, the synaptic protein neurexin-1, to which CASK is recruited via its PDZ domain. Thus, our data indicate that CASK combines the scaffolding activity of MAGUKs with an unusual kinase activity that phosphorylates substrates recuited by the scaffolding activity. Moreover, our study suggests that other pseudokinases (10% of the kinome) could also be catalytically active.

CASK mutations are frequent in males and cause X-linked nystagmus and variable XLMR phenotypes

“…The CaMK-like domain binds to several proteins, such as Caskin 1 21 and Mint1, 22,23 and adopts a constitutively active conformation that binds to ATP and catalyses the transfer of a phosphate group without Mg2 + . 24 The lack of sequence variants of the missense mutations in control samples also suggests that the evolutionary constraints on CASK are stringent and that these mutations are unlikely to be polymorphisms, but this cannot be completely excluded.…”

Molecular cloning and characterization ofNcROP2Fam-1, a member of the ROP2 family of rhoptry proteins inNeospora caninumthat is targeted by antibodies neutralizing host cell invasionin vitro

“…NcROP2Fam-1 can therefore be qualified as a pseudokinase (Boudeau et al 2006). However, it would be difficult to make any assumption as to whether NcROP2Fam-1 might nevertheless have a kinase activity since a number of pseudokinases, such as WNKs (with-no-lysine (K)) or CASK (Ca 2 + /calmodulin-dependent serine kinase) have evolved alternative ways of binding ATP and performing the phosphoryl transfer reaction (Xu et al 2000;Mukherjee et al 2008).…”

Structural analysis of the catalytically inactive kinase domain of the human EGF receptor 3

“…Pseudokinases, which represent Ϸ10% of all human kinases, are assumed to act as scaffolds or allosteric regulators rather than enzymes and are still relatively poorly characterized as a group. The structures of two pseudokinase domains have been reported so far, those of Ca 2ϩ /calmodulin-activated Ser-Thr kinase (CASK) (13) and vaccinia-related kinase 3 (VRK3) (14).…”