Untangling Flavobacterium johnsoniae Gliding Motility and Protein Secretion.

Flavobacterium johnsoniae exhibits rapid gliding motility over surfaces. At least 20 genes are involved in this process. Seven of these, gldK, gldL, gldM, gldN, sprA, sprE, and sprT, encode proteins of the type IX protein secretion system (T9SS). The T9SS is required for surface localization of the motility adhesins SprB and RemA, and for secretion of the soluble chitinase ChiA. Here, we demonstrate that the gliding motility proteins GldA, GldB, GldD, GldF, GldH, GldI, and GldJ are also essential for secretion. Cells with mutations in the genes encoding any of these seven proteins had normal levels of gldK mRNA but dramatically reduced levels of the GldK protein, which may explain the secretion defects of the motility mutants. GldJ is necessary for stable accumulation of GldK, and each mutant lacked the GldJ protein. F. johnsoniae cells that produced truncated GldJ, lacking eight to 13 amino acids from the C terminus, accumulated GldK but were deficient in gliding motility. SprB was secreted by these cells but was not propelled along their surfaces. This C-terminal region of GldJ is thus required for gliding motility but not for secretion. The identification of mutants that are defective for motility but competent for secretion begins to untangle the F. johnsoniae gliding motility machinery from the T9SS.IMPORTANCE Many members of the phylum Bacteroidetes secrete proteins using T9SSs. T9SSs appear to be confined to members of this phylum. Many of these bacteria also glide rapidly over surfaces using a motility machine that is also confined to the Bacteroidetes and appears to be intertwined with the T9SS. This study identifies F. johnsoniae proteins that are required for both T9SS function and gliding motility. It also provides an explanation for the link between secretion and gliding and identifies mutants with defects in motility but not secretion.