Prevalent association with the bacterial cell envelope of prokaryotic expansins revealed by bioinformatics analysis

Expansins are a group of proteins from diverse organisms from bacteria to plants. Although expansins show structural conservation, their biological roles seem to differ among kingdoms. In plants, these proteins remodel the cell wall during plant growth and other processes. Contrarily, determination of bacterial expansin activity has proven difficult, although genetic evidence of bacterial mutants indicates that expansins participate in bacteria-plant interactions. Nevertheless, a large proportion of expansin genes are found in the genomes of free-living bacteria, suggesting roles that are independent of the interaction with living plants. Here, we analyzed all available sequences of prokaryotic expansins for correlations between surface electric charge, extra protein modules, and sequence motifs for association with the bacteria exterior after export. Additionally, information on the fate of protein after translocation across the membrane also points to bacterial cell association of expansins through six different mechanisms, such as attachment of a lipid molecule for membrane anchoring in diderm species or covalent linking to the peptidoglycan layer in monoderms such as the Bacilliales. Our results have implications for expansin function in the context of bacteria-plant interactions and also for free-living species in which expansins might affect cell-cell or cell-substrate interaction properties and indicate the need to re-examine the roles currently considered for these proteins.

Automated summary

Expansins are a group of proteins found in various organisms, including bacteria and plants, with different roles among different kingdoms. In plants, they remodel the cell wall, while their function in bacteria is challenging to determine. However, genetic evidence from bacterial mutants suggests their involvement in interactions with plants, and the presence of expansin genes in free-living bacteria indicates independent roles. This analysis provides insights into the association of bacterial expansins with the bacterial cell exterior and their potential impact on bacteria-plant interactions and other cell-cell or cell-substrate interactions in free-living species, suggesting a re-evaluation of the current understanding of expansin functions.