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

Protein export by Gram-negative bacteria requires devoted machineries to al low for the passage of hydrolytic enzymes and toxins through the cell envel ope. The Type II export machinery has a Number of distinct characteristics, which include ifs role as an extension of Sec-dependent secretion, ifs abi lity to recognize and export fully folded substrates efficiently and, perha ps wrest significantly, the relationship between a subset of its gene produ cts with the Type IV pilus-biogenesis apparatus. An important question is w hether we can extrapolate our knowledge, albeit limited, of Type IV pilus b iogenesis to understand the structure and function of the Type Il export ap paratus. This and other questions relating to the energetics of assembly an d specificity of the apparatus care addressed in this article.