The C-terminal domain of the secretin PulD contains the binding site for its cognate chaperone, PulS, and confers PulS dependence on plV(f1) function

Related outer membrane proteins, termed secretins, participate in the secretion of macromolecules across the outer membrane of many Gram-negative bacteria. In the pullulanase-secretion system, PulS, an outer membrane-associated lipoprotein, is required both for the integrity and the proper outer membrane localization of the PulD secretin. Here we show that the PulS-binding site is located within the C-terminal 65 residues of PulD. Addition of this domain to the filamentous phage secretin, plV, or to the unrelated maltose-binding protein rendered both proteins dependent on PulS for stability. A chimeric protein composed of bacteriophage f1 plV and the C-terminal domain of PulD required properly localized PuIS to support phage assembly. An in vivo complex formed between the plV-PulD(65) chimera and PulS was detected by co-immunoprecipitation and by affinity chromatography.