The N- and C-terminal domains of MecA recognize different partners in the competence molecular switch.

ComK is a transcription factor required for the expression of competence genes in Bacillus subtilis. Binding to MecA targets ComK for degradation by the ClpCP protease. MecA therefore acts as an adapter protein recruiting a regulatory protein for proteolysis. However, when ComS is synthesized, ComK is released from binding by MecA and thereby protected from degradation. MecA binds to three protein partners during these processes: ComK, ClpC and ComS. Using limited proteolysis, we have defined N- and C-terminal structural domains of MecA and evaluated the interactions of these domains with the protein partners of MecA. Using surface plasmon resonance, we have determined that the N-terminal domain of MecA interacts with ComK and ComS and the C-terminal domain with ClpC. MecA is shown to exist as a dimer with dimerization sites on both the N- and C-terminal domains. The C-terminal domain stimulates the ATPase activity of ClpC and is degraded by the ClpCP protease, while the N-terminal domain is inactive in both of these assays. In vivo data were consistent with these findings, as comG-lacZ expression was decreased in a strain overproducing the N-terminal domain, indicating reduced ComK activity. We propose a model in which binding of ClpC to the C-terminal domain of MecA induces a conformational change enabling the N-terminal domain to bind ComK with enhanced affinity. MecA is widespread among Gram-positive organisms and may act generally as an adapter protein, targeting proteins for regulated degradation.