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

FEMS Microbiol Rev. 2000 Jan;24(1):21-44. doi: 10.1111/j.1574-6976.2000.tb00531.x.


Bacteria synthesize large-sized surface structures through the ordered polymerization of protein subunits. This results in planar or tubular regular structures that have evolved to accomplish specific functions related to the particular environment in which these bacteria are found. Tubular assemblies 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. Pili or fimbriae are less complicated helical filaments, which consist of a major subunit and 3-5 minor subunits or pilins, whose main function is the attachment to specific surfaces. Planar structures known as S-layers are the simplest of these regular assemblies and are generally made up of a single subunit packed as a bidimensional crystal around the whole cell surface. Most of the components of these structures have to be secreted through the inner membrane (IM), the periplasm and the outer membrane (OM) before reaching their final destination. The so called general secretory pathway (GSP), or type II secretion system, appears to be implicated in this process to varying degrees, depending on the structure considered. A few S-layers and pili require GSP components but also 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 identified in some pathogens. Moreover, secretion of subunits of S-layers using dedicated type I machinery, without the involvement of any GSP component, has also been observed.

Publication types

  • Review

MeSH terms

  • Amino Acid Sequence
  • Bacterial Outer Membrane Proteins / genetics
  • Bacterial Outer Membrane Proteins / metabolism
  • Bacterial Outer Membrane Proteins / physiology
  • Carrier Proteins / metabolism
  • Cell Membrane / metabolism
  • Fimbriae, Bacterial / genetics
  • Fimbriae, Bacterial / metabolism
  • Fimbriae, Bacterial / ultrastructure
  • Flagella / genetics
  • Flagella / metabolism
  • Flagella / ultrastructure
  • Flagellin / genetics
  • Flagellin / metabolism
  • Gram-Negative Bacteria / genetics
  • Gram-Negative Bacteria / metabolism*
  • Gram-Negative Bacteria / ultrastructure*
  • Molecular Sequence Data
  • Sequence Homology, Amino Acid


  • Bacterial Outer Membrane Proteins
  • Carrier Proteins
  • Flagellin