Cell surface localization and processing of the ComG proteins, required for DNA binding during transformation of Bacillus subtilis

The comG operon of Bacillus subtilis encodes seven proteins essential for the binding of transforming DNA to the competent cell surface. We have explored the processing of the ComG proteins and the cellular localization of six of them. All of the proteins were found to be membrane associated. The four proteins with N-terminal sequence motifs typical of type 4 pre-pilins (ComGC, GD, GE and GG) are processed by a pathway that requires the product of comC, also an essential competence gene. The unprocessed forms of ComGC and GD behave like integral membrane proteins. Pre-ComGG differs from pre-ComGC and pre-ComGD, in that it is accessible to proteolysis only from the cytoplasmic face of the membrane and at least a portion of it behaves like a peripheral membrane protein. The mature forms of these proteins are translocated to the outer face of the membrane and are liberated when peptidoglycan is hydrolysed by lysozyme or mutanolysin. ComGG exists in part as a disulphide-cross-linked homodimer in vivo. ComGC was found to possess an intramolecular disulphide bond. The previously identified homodimer form of this protein is not stabilized by disulphide bond formation. ComGF behaves as an integral membrane protein, while ComGA, a putative ATPase, is located on the inner face of the membrane as a peripheral membrane protein. Possible roles of the ComG proteins in DNA binding to the competent cell surface are discussed in the light of these and other results.