Secretion and assembly of regular surface structures in Gram-negative bacteria

Bacteria synthesize large-sized surface structures through the ordered poly merization of protein subunits. This results in planar or tubular regular s tructures that have evolved to accomplish specific functions related to the particular environment in which these bacteria are found. Tubular assembli es known as flagella are the most complex structures known in bacteria and consist of a helical rigid filament, a torsion adapter or hook and a proton -fueled rotator known as the basal body. Pill or fimbriae are less complica ted helical filaments, which consist of a major subunit and 3-5 minor subun its or pilins, whose main function is the attachment to specific surfaces. Planar structures known as S-layers are the simplest of these regular assem blies and are generally made up of a single subunit packed as a bidimension al crystal around the whole cell surface. Most of the components of these s tructures have to be secreted through the inner membrane (IM), the periplas m and the outer membrane (OM) before reaching their final destination. The so called general secretory pathway (GSP), or type II secretion system, app ears to be implicated in this process to varying degrees, depending on the structure considered. A few S-layers and pill require GSP components but al so need specific terminal branches. such as the well known chaperone-usher pathway. On the other hand, only two of the nearly 40 proteins involved in flagellar assembly are dependent on the: GSP, while the external components are secreted through a specific pathway similar to the type III systems id entified in some pathogens. Moreover, secretion of subunits of S-layers usi ng dedicated type I machinery, without the involvement of any GSP component , has also been observed. (C) 2000 Federation of European Microbiological S ocieties. Published by Elsevier Science B.V. All rights reserved.