Assembly of complex organelles: Pilus biogenesis in Gram-negative bacteriaas a model system

Pathogenic bacteria assemble a variety of adhesive structures on their surf ace for attachment to host cells. Some of these structures are quite comple x. For example, the hair-like organelles known as pill or fimbriae are gene rally composed of several components and often exhibit composite morphologi es. In Gram-negative bacteria assembly of pill requires that the subunits c ross the cytoplasmic membrane, fold correctly in the periplasm, target to t he outer membrane, assemble into an ordered structure, and cross the outer membrane to the cell surface. Thus, pilus biogenesis provides a model for a number of basic biological problems including protein folding, trafficking , secretion, and the ordered assembly of proteins into complex structures, P pilus biogenesis represents one of the best-understood pilus systems. P p ill are produced by 80-90% of all pyelonephritic Escherichia coli and are a major virulence determinant for urinary tract infections. Two specialized assembly factors known as the periplasmic chaperone and outer membrane ushe r are required for P pilus assembly. A chaperone/usher pathway is now known to be required for the biogenesis of more than 30 different adhesive struc tures in diverse Gram-negative pathogenic bacteria. Elucidation of the chap erone/usher pathway was brought about through a powerful combination of mol ecular, biochemical, and biophysical techniques, This review discusses thes e approaches as they relate to pilus assembly, with an emphasis on newer te chniques. (C) 2000 Academic Press.