An extended hydrophobic interactive surface of Yersinia pestis Caf1M chaperone is essential for subunit binding and F1 capsule assembly

A single polypeptide subunit, Caf1, polymerizes to form a dense, poorly defined structure (F1 capsule) on the surface of Yersinia pestis. The caf‐encoded assembly components belong to the chaperone–usher protein family involved in the assembly of composite adhesive pili, but the Caf1M chaperone itself belongs to a distinct subfamily. One unique feature of this subfamily is the possession of a long, variable sequence between the F1 β‐strand and the G1 subunit binding β‐strand (FGL; F1 β‐strand to G1 β‐strand long). Deletion and insertion mutations confirmed that the FGL sequence was not essential for folding of the protein but was absolutely essential for function. Site‐specific mutagenesis of individual residues identified Val‐126, in particular, together with Val‐128 as critical residues for the formation of a stable subunit–chaperone complex and the promotion of surface assembly. Differential effects on periplasmic polymerization of the subunit were also observed with different mutants. Together with the G1 strand, the FGL sequence has the potential to form an interactive surface of five alternating hydrophobic residues on Caf1M chaperone as well as in seven of the 10 other members of the FGL subfamily. Mutation of the absolutely conserved Arg‐20 to Ser led to drastic reduction in Caf1 binding and surface assembled polymer. Thus, although Caf1M–Caf1 subunit binding almost certainly involves the basic principle of donor strand complementation elucidated for the PapD–PapK complex, a key feature unique to the chaperones of this subfamily would appear to be capping via high‐affinity binding of an extended hydrophobic surface on the respective single subunits.