Characterization of the Interaction of Phorbol Esters with the C1 Domain of MRCK (Myotonic Dystrophy Kinase-related Cdc42 Binding Kinase) α/β.

Cl domains mediate the recognition and subsequent signaling response to diacylglycerol and phorbol esters by protein kinase C (PKC) and by several other families of signal-transducing proteins such as the chimerins or RasGRP. MRCK (myotonic dystrophy kinase-related Cdc42 binding kinase), a member of the dystrophia myotonica protein kinase family that functions downstream of Cdc42, contains a Cl domain with substantial homology to that of the diacylglycerol/phorbol ester-responsive Cl domains and has been reported to bind phorbol ester. We have characterized here the interaction of the Cl domains of the two MRCK isoforms α and β with phorbol ester. The MRCK C1 domains bind [20-³H]phorbol 12,13-dibutyrate with Kd values of 10 and 17 nM, respectively, reflecting 60-90-fold weaker affinity compared with the protein kinase C δ C1b domain. In contrast to binding by the C1b domain of PKCδ, the binding by the C1 domains of MRCK α and β was fully dependent on the presence of phosphatidylserine. Comparison of ligand binding selectivity showed resemblance to that by the C1b domain of PKCα and marked contrast to that of the C1b domain of PKCδ. In intact cells, as in the binding assays, the MRCK C1 domains required 50-100-fold higher concentrations of phorbol ester for induction of membrane translocation. We conclude that additional structural elements within the MRCK structure are necessary if the C1 domains of MRCK are to respond to phorbol ester at concentrations comparable with those that modulate PKC. [ABSTRACT FROM AUTHOR]