The extracellular nucleation-precipitation (ENP) pathway or Type VIII secretion system (T8SS) in Gram-negative (diderm) bacteria is responsible for the secretion and assembly of prepilins for fimbiae biogenesis, the prototypical curli. Besides the T2SS that can be involved in the assembly of prototypical Type 4 pilus, the T4SS that can be involved in the biogenesis of the prototypical pilus T, the T3SS involved in the assembly of the injectisome and the T7SS involved in the formation of the prototypical Type 1 pilus, the T8SS differs in that fibre-growth occurs extracellularly. The curli, also called thin aggregative fimbriae (Tafi), are the only fimbriae dependent on the T8SS. Tafi were first identified in Salmonella spp and the controlling operon termed agf; however subsequent isolation of the homologous operon in E coli led to its being called csg. In the absence of extracellular polysaccharides Tafi appear curled, although when expressed with such polysaccharides their morphology appears as a tangled amorphous matrix [2]. CsgF is one of three putative curli assembly factors appearing to act as a nucleator protein. Unlike eukaryotic amyloid formation, curli biogenesis is a productive pathway requiring a specific assembly machinery [1].

1: Role of Escherichia coli curli operons in directing amyloid fiber formation. Chapman MR, Robinson LS, Pinkner JS, Roth R, Heuser J, Hammar M, Normark S, Hultgren SJ; Science. 2002;295:851-855. PMID:11823641

2: AgfC and AgfE facilitate extracellular thin aggregative fimbriae synthesis in Salmonella enteritidis. Gibson DL, White AP, Rajotte CM, Kay WW; Microbiology. 2007;153:1131-1140. PMID:17379722

3: The general secretory pathway: a general misnomer?. Desvaux M, Parham NJ, Scott-Tucker A, Henderson IR; Trends Microbiol. 2004;12:306-309. PMID:15223057

4: Type II protein secretion and its relationship to bacterial type IV pili and archaeal flagella. Peabody CR, Chung YJ, Yen MR, Vidal-Ingigliardi D, Pugsley AP, Saier MH Jr; Microbiology. 2003;149:3051-3072. PMID:14600218

5: Secretion and subcellular localizations of bacterial proteins: a semantic awareness issue. Desvaux M, Hebraud M, Talon R, Henderson IR; Trends Microbiol. 2009;17:139-145. PMID:19299134