The Peptidoglycan-Binding Protein FimV Promotes Assembly of the
Pseudomonas aeruginosa
Type IV Pilus Secretin
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ABSTRACT
The
Pseudomonas aeruginosa
inner membrane protein FimV is among several proteins of unknown function required for type IV pilus-mediated twitching motility, arising from extension and retraction of pili from their site of assembly in the inner membrane. The pili transit the periplasm and peptidoglycan (PG) layer, ultimately exiting the cell through the PilQ secretin. Although
fimV
mutants are nonmotile, they are susceptible to killing by pilus-specific bacteriophage, a hallmark of retractable surface pili. Here we show that levels of recoverable surface pili were markedly decreased in
fimV pilT
retraction-deficient mutants compared with levels in the
pilT
control, demonstrating that FimV acts at the level of pilus assembly. Levels of inner membrane assembly subcomplex proteins PilM/N/O/P were decreased in
fimV
mutants, but supplementation of these components in
trans
did not restore pilus assembly or motility. Loss of FimV dramatically reduced the levels of the PilQ secretin multimer through which pili exit the cell, in part due to decreased levels of PilQ monomers, while PilF pilotin levels were unchanged. Expression of
pilQ
in
trans
in the wild type or
fimV
mutants increased total PilQ monomer levels but did not alter secretin multimer levels or motility. PG pulldown assays showed that the N terminus of FimV bound PG in a LysM motif-dependent manner, and a mutant with an in-frame chromosomal deletion of the LysM motif had reduced motility, secretin levels, and surface piliation. Together, our data show that FimV's role in pilus assembly is to promote secretin formation and that this function depends upon its PG-binding domain.
