Characterization of F-pilin as an inner membrane component of Escherichia coli K12

Antipeptide antibodies were used to detect, purify, and characterize nonfilament F-pilin in the cell envelope of an F′tra⁺ strain of Escherichia coli. Affinity-purified goat antibodies raised against a peptide corresponding to the ammo-terminal 14 arnino acids of F-pilin detected F-pilin in immuno-overlay ("Western") blots of electrophoretically separated inner and outer membrane proteins. As expected, the molecule was absent from inner membrane preparations of F^- or F′traA[Am] strains. Immunoreactive material was purified from inner membrane fractions and shown to be F-pilin by amino acid analysis. The anti-peptide antibodies also detected membrane forms of F-pilin produced by cells containing plasmid pTG801 (Grossman, T. & Silverman, P. (1989) J. Bacteriol. 171, 650-656). Most cell envelope pilin was in the inner membrane fraction, but a significant quantity fractionated with the outer membrane as well. The hydropathy profile of F-pilin suggested that the molecule is an integral membrane protein with two membrane-spanning domains. In confirmation, F-pilin and pTG801 pilins in inner membrane preparations were solubilized by a single extraction with the nonionic detergents Nonidet P-40 (2%) or Triton X-100 (2%), but not by 2 M KCl or 0.1 M NaOH. Moreover, analysis of traA′-′phoA constructs indicated that both the amino and carboxyl termini of F-pilin face the periplasm. The periplasmic location of the amino terminus was confirmed by immunoelectron microscopy of spheroplasts from F′ and pTG801 strains, using a monoclonal antibody that recognizes an amino-terminal epitope. These data suggest a specific structure for membrane F-pilin. We discuss that structure in relation to the probable structure of filament F-pilin.