Regulatory domains of ca²⁺/calmodulin-dependent protein kinases

Skeletal and smooth muscle myosia light chain kinases (SkMLCK and SmMLCK) and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) catalytic cores are regulated intrasterically by respective autoinhibitory and calmodulin (CaM) binding sequence. We have investigated the functional properties of these regulatory segments by expressing chimeric catalytic cores containing different regulatory segments by (denoted in [ ]). SkMLCK and SkMLCK[SmMLCK] had similar activities (V max/Km values) towards regulatory light chain from smooth (SmRLC) and skeletal (SmRLC) muscles. and similar CaM activation properties (Kc _aM = 1nM). SkMLCK[CaMKII] also had a similar KCM value but had a 7-fold decrease and undeterminable Vmax/Km values for SmRLC and SkRLC, respectivlely, SkMLcK[nNOS] (neurolnal nitric oxide synthase CaM binidng domain) had no activity although it was expressed. In comparison to CaMKII, CaMKII [SkMLCK] and CaMKII had similar high KCam Values (26-50 nM), but Vmax/Km values for SmRLC were 10-20 fold lower. All kinases bound Cam. except for SkMLCK[nNOS], chimeric activities wee Ca²⁺/Cam-dependent and were convered to Ca²⁺/CaM-independent activity after trysin digestion. Thus, but erologous regulatory segments affect substrate recognition and kinase activity. Furthermore, the sensitivity to CaM activation is determined primarliy by the respective catalytic cores. not the CaM binding segments for SkMLCK and CaMKII. (supported in part by the Bashour Research Fund and NIH Grants HL26093 and HL 06296).