Bacterial Type II protein export and pilus biogenesis: more than just homologies?

Protein export by Gram-negative bacteria requires devoted machineries to allow for the passage of hydrolytic enzymes and toxins through the cell envelope. The Type II export machinery has a Number of distinct characteristics, which include ifs role as an extension of Sec-dependent secretion, ifs ability to recognize and export fully folded substrates efficiently and, perhaps wrest significantly, the relationship between a subset of its gene products with the Type IV pilus-biogenesis apparatus. An important question is whether we can extrapolate our knowledge, albeit limited, of Type IV pilus biogenesis to understand the structure and function of the Type Il export apparatus. This and other questions relating to the energetics of assembly and specificity of the apparatus care addressed in this article.