# Functional characterization of the polar organizer protein FimV in Pseudomonas putida

## TL;DR

The study explores the role of the polar organizer protein FimV in Pseudomonas putida and finds it has limited functional overlap with similar proteins in other bacteria.

## Contribution

The paper reveals that Pseudomonas putida FimV has distinct functions compared to homologous proteins in other species, highlighting functional diversity.

## Key findings

- PpFimV binds to peptidoglycan via its LysM domain and is necessary for polar positioning.
- PpFimV is involved in flagellar-mediated swimming and chemotaxis but not in chromosome segregation or pilus-dependent motility.
- PpFimV and SpHubP cannot compensate for each other in soft agar swimming, indicating distinct functional roles.

## Abstract

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 Pseudomonas putida FimV (PpFimV) with respect to cellular processes involving FimV or HubP in other species. We found that PpFimV nonspecifically binds to peptidoglycan via its periplasmic LysM domain, which, together with an immunoglobulin-like domain, is necessary for proper polar positioning. PpFimV is required for normal flagellar-mediated swimming and the placement of the chemotaxis system. However, PpFimV is not involved in regulating the number of flagellar filaments, chromosome segregation, or type IV pilus-dependent surface motility. Thus, PpFimV has surprisingly little functional overlap with, for example, HubP from Vibrio sp. or Shewanella putrefaciens or with FimV from P. aeruginosa. PpFimV was unable to compensate for the loss of SpHubP with regard to swimming in soft agar, and vice versa. Domain swapping between SpHubP and PpFimV 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.

Many 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 Pseudomonas putida FimV and Shewanella putrefaciens HubP. This finding suggests a significant degree of functional diversity.