Structural and functional significance of the FGL sequence of the periplasmic chaperone XXXX M of @entity12502 .

@entity52477 @entity33354 @entity535 @entity71989 @entity52476 @entity12502 @entity526
The periplasmic molecular chaperone @entity71989 of @entity12502 is a typical representative of a subfamily of specific chaperones involved in assembly of surface adhesins with a very simple structure. One characteristic feature of this @entity71989 -like subfamily is possession of an extended, variable sequence (termed FGL) between the F1 and subunit binding G1 beta-strands. In contrast, @entity33354 subfamily members, characterized by PapD, have a short F1-G1 loop and are involved in assembly of complex pili. To elucidate the structural and functional significance of the FGL sequence, a mutant @entity71989 molecule (dCaf1M), in which the 27 @entity535 residues between the F1 and G1 beta-strands had been deleted, was constructed. Expression of the mutated @entity71989 in @entity526 resulted in accumulation of high levels of dCaf1M. The far-UV circular dichroism spectra of the mutant and wild-type proteins were indistinguishable and exhibited practically the same temperature and pH dependencies. Thus, the FGL sequence of @entity71989 clearly does not contribute significantly to the stability of the protein conformation. Preferential cleavage of @entity71989 by trypsin at Lys-119 confirmed surface exposure of this part of the FGL sequence in the isolated chaperone and periplasmic chaperone-subunit complex. There was no evidence of surface-localized @entity52477 subunit in the presence of the @entity52476 outer membrane protein and dCaf1M. In contrast to @entity71989 , dCaf1M was not able to form a stable complex with @entity52477 nor could it protect the subunit from proteolytic degradation in vivo. This demonstration that the FGL sequence is required for stable chaperone-subunit interaction, but not for folding of a stable chaperone, provides a sound basis for future detailed molecular analyses of the FGL subfamily of chaperones.
