Chaperone-independent folding of type 1 pilus domains

An elementary step in the assembly of adhesive type 1 pili of Escherichia coli is the folding of structural pilus subunits in the periplasm. The previously determined X-ray structure of the complex between the type 1 pilus adhesin FimH and the periplasmic pilus assembly chaperone FimC has shown that FimH consists of a N-terminal lectin domain and a C-terminal pilin domain, and that FimC exclusively interacts with the pilin domain. The pilin domain fold, which is common to all pilus subunits, is characterized by an incomplete beta-sheet that is completed by a donor strand from FimC in the FimC-FimH complex. This, together with unsuccessful attempts to refold isolated, urea-denatured FimH in vitro had suggested that folding of pilin domains strictly depends on sequence information provided by FimC. We have now analyzed in detail the folding of FimH and its two isolated domains in vitro. We find that not only the lectin domain, but also the pilin domain can fold autonomously and independently of FimC. However, the thermodynamic stability of the pilin domain is very low (8-10kJmol(-1)) so that a significant fraction of the domain is unfolded even in the absence of denaturant. This explains the high tendency of structural pilus subunits to aggregate non-specifically in the absence of stoichiometric amounts of FimC. Thus, pilus chaperones prevent non-specific aggregation of pilus subunits by native state stabilization after subunit folding.