Identification, subcellular localization and functional interactions of PilMNOWQ and PilA4 involved in transformation competency and pilus biogenesis in the thermophilic bacterium Thermus thermophilus HB27

The natural transformation system of the thermophilic bacterium Thermus thermophilus HB27 comprises at least 16 distinct competence proteins encoded by seven distinct loci. In this article, we present for the first time biochemical analyses of the Thermus thermophilus competence proteins PilMNOWQ and PilA4, and demonstrate that the pilMNOWQ genes are each essential for natural transformation. We identified three different forms of PilA4, one with an apparent molecular mass of 14 kDa, which correlates with that of the deduced protein, an 18-kDa form and a 23-kDa form; the last was found to be glycosylated. We demonstrate that PilM, PilN and PilO are located in the inner membrane, whereas PilW, PilQ and PilA4 are located in the inner and outer membranes. These data show that PilMNOWQ and PilA4 are components of a DNA translocator structure that spans the inner and outer membranes. We further show that PilA4 and PilQ both copurify with pilus structures. Possible functions of PilQ and PilA4 in DNA translocation and in pilus biogenesis are discussed. Comparative mutant studies revealed that mutations in either pilW or pilQ significantly affect the location of the other protein in the outer membrane. Furthermore, no PilA4 was present in the outer membranes of these mutants. From these findings, we conclude that the abilities of PilW, PilQ and PilA4 to stably localize or accumulate in the outer membrane fraction are strongly dependent on one another, which is in accord with an outer membrane DNA translocator complex comprising PilW, PilQ, and PilA4.