Functional characterization of the polar organizer protein FimV in <i>Pseudomonas putida</i>

Homologs of the polar landmark proteins HubP and FimV are widespread among bacterial species. They all share several common features, including a periplasmic LysM-like domain, a transmembrane region, an extensive cytoplasmic domain enriched in acidic amino acids, and a C-terminal tetrapeptid-repeat (TPR) domain referred to as the FimV domain. Apart from these conserved general features, however, the proteins exhibit little homology across different bacterial genera. Here, we characterized <i>Pseudomonas putida</i> FimV (<i>Pp</i>FimV) with respect to cellular processes involving FimV or HubP in other species. We found that <i>Pp</i>FimV nonspecifically binds to peptidoglycan via its periplasmic LysM domain, which, together with an immunoglobulin-like domain, is necessary for proper polar positioning. <i>Pp</i>FimV is required for normal flagellar-mediated swimming and the placement of the chemotaxis system. However, <i>Pp</i>FimV is not involved in regulating the number of flagellar filaments, chromosome segregation, or type IV pilus-dependent surface motility. Thus, <i>Pp</i>FimV has surprisingly little functional overlap with, for example, HubP from <i>Vibrio</i> sp. or <i>Shewanella putrefaciens</i> or with FimV from <i>P. aeruginosa. Pp</i>FimV was unable to compensate for the loss of <i>Sp</i>HubP with regard to swimming in soft agar, and <i>vice versa</i>. Domain swapping between <i>Sp</i>HubP and <i>Pp</i>FimV revealed that differences in the cytoplasmic region between the transmembrane region and the C-terminal FimV domain likely account for the proteins' distinct functions in flagella-mediated swimming. This suggests that FimV and HubP are structural homologs that have evolved to perform different, species-specific functions.IMPORTANCEMany bacterial species possess landmark proteins that organize the bacterial cell and localize specific cellular processes to the cell's polar regions by directing client proteins or protein complexes to their designated positions. FimV and its homolog HubP are landmark proteins found in many species of the gammaproteobacteria, but their roles are not well understood. Here, we demonstrate that only certain functions related to flagella-mediated motility appear to be conserved between <i>Pseudomonas putida</i> FimV and <i>Shewanella putrefaciens</i> HubP. This finding suggests a significant degree of functional diversity.