Cohesive Properties of the Caulobacter crescentus Holdfast Adhesin Are Regulated by a Novel c-di-GMP Effector Protein

Abstract

When encountering surfaces, many bacteria produce adhesins to facili- tate their initial attachment and to irreversibly glue themselves to the solid sub- strate. A central molecule regulating the processes of this motile-sessile transition is the second messenger c-di-GMP, which stimulates the production of a variety of ex- opolysaccharide adhesins in different bacterial model organisms. In Caulobacter cres- centus , c-di-GMP regulates the synthesis of the polar holdfast adhesin during the cell cycle, yet the molecular and cellular details of this control are currently unknown. Here we identify HfsK, a member of a versatile N -acetyltransferase family, as a novel c-di-GMP effector involved in holdfast biogenesis. Cells lacking HfsK form highly mal- leable holdfast structures with reduced adhesive strength that cannot support sur- face colonization. We present indirect evidence that HfsK modifies the polysaccha- ride component of holdfast to buttress its cohesive properties. HfsK is a soluble protein but associates with the cell membrane during most of the cell cycle. Coinci- dent with peak c-di-GMP levels during the C. crescentus cell cycle, HfsK relocalizes to the cytosol in a c-di-GMP-dependent manner. Our results indicate that this c-di-GMP- mediated dynamic positioning controls HfsK activity, leading to its inactivation at high c-di-GMP levels. A short C-terminal extension is essential for the membrane as- sociation, c-di-GMP binding, and activity of HfsK. We propose a model in which c-di- GMP binding leads to the dispersal and inactivation of HfsK as part of holdfast bio- genesis progression.