MACROMOLECULAR ASSEMBLY AND SECRETION ACROSS THE BACTERIAL-CELL ENVELOPE - TYPE-II PROTEIN SECRETION SYSTEMS

A decade ago, Pugsley and colleagues reported the existence of a large region of Klebsiella DNA, distinct from the Klebsiella gene encoding pullulanase, which was necessary for secretion of this enzyme to the c ell surface in Escherichia coli (d'Enfert et al., 1987a,b). The pul ge nes it contained proved to be the tip of an iceberg. The sequences rep orted before 1992 (d'Enfert et al., 1987a,b; d'Enfert & Pugsley, 1989; Pugsley & Reyss, 1990; Reyss & Pugsley, 1990) included only one gene (pulD) that matched any sequence in the data base; a 220 aminoacid res idue segment of PulD was 32% identical with a portion of the filamento us phage-encoded protein, pIV. But by the time the sequence of the 18. 8 kb DNA fragment that contained the pul genes had been completed (Pos sot et al., 1992), reports of sets of homologous genes in several spec ies of Gram-negative plant and animal pathogens had appeared. For the most part, these gene clusters were cloned by their ability to complem ent mutants that produced, but failed to secrete, proteins normally fo und in the extracellular milieu; when tested, the mutants showed reduc ed pathogenicity or were totally avirulent. The secreted proteins incl uded hydrolytic enzymes such as cellulase and pectinase from plant pat hogens, and proteases and toxins from animal pathogens. The multi-gene family necessary for secretion of these enzymes is now known as the t ype II system or the main terminal branch (MTB) of the general secreti on pathway (GSP). As summarized by Pugsley ef al. (1997), the current tally includes type II systems from Klebsiella oxytoca (pul), Erwinia chrysanthemi and carotovora (out), Xanthomonas campestris (xps), Pseud omonas aeruginosa (xcp), Aeromonas hydrophila (exe), and Vibrio choler ae (eps). A second type II system (sps) necessary for deposition of th e S-layer on the cell surface in A. hydrophila is more similar to the X. campestris than A. hydrophila genes (Thomas & Trust, 1995). The big gest surprise has been the discovery of a complete set of type II secr etion genes in E. coli K12. The E. coli genes are not expressed under normal growth conditions, and a search is underway to find inducing co nditions and secretion substrates (Francetic & Pugsley, 1996). Impress ive progress has already been made in defining components of the pathw ay. What remains to be understood in mechanistic detail is how this pr otein secretion system functions. (C) 1998 Academic Press Limited.