Flavobacterium johnsoniae RemA is a mobile cell surface lectin involved in gliding

Cells of Flavobacterium johnsoniae move rapidly over surfaces by a process known as gliding motility. Gld proteins are thought to comprise the motor that propels the cell surface adhesin SprB. Cells with mutations in sprB are partially defective in motility and are also resistant to some bacteriophages. Transposon mutagenesis of a strain carrying a deletion spanning sprB identified eight mutants that were resistant to additional phages and exhibited reduced motility. Four of the mutants had transposon insertions in remA, which encodes a cell surface protein that has a lectin domain and appears to interact with polysaccharides. Three other genes identified in this screen (remC, wza, and wzc) encode proteins predicted to be involved in polysaccharide synthesis and secretion. Myc-tagged versions of RemA localized to the cell surface and were propelled rapidly along the cell at speeds of 1 to 2 μm/s. Deletion of gldN and gldO, which encode components of a bacteroidete protein secretion system, blocked the transport of RemA to the cell surface. Overexpression of RemA resulted in the formation of cell aggregates that were dispersed by the addition of galactose or rhamnose. Cells lacking RemC, Wza, and Wzc failed to aggregate. Cells of a remC mutant and cells of a remA mutant, neither of which formed aggregates in isolation, aggregated when they were mixed together, suggesting that polysaccharides secreted by one cell may interact with RemA on another cell. Fluorescently labeled lectin Ricinus communis agglutinin I detected polysaccharides secreted by F. johnsoniae. The polysaccharides bound to cells expressing RemA and were rapidly propelled on the cell surface. RemA appears to be a mobile cell surface adhesin, and secreted polysaccharides may interact with the lectin domain of RemA and enhance motility.