Stimulation of the Caulobacter crescentus surface sensing pathway by deletion of a specialized minor pilin-like gene

Bacteria colonize surfaces through complex mechanisms of surface sensing. Pili are dynamic bacterial appendages that play an important role in this process. In Caulobacter crescentus , tension on retracting, surface-bound pili triggers the rapid synthesis of the adhesive holdfast, which permanently attaches cells to surfaces. However, the detailed mechanisms of pilus-mediated surface sensing are unclear. In this study, we used a genetic screen to isolate mutants with altered pilus activity to identify genes that may be involved in pilus-mediated surface-sensing. This screen identified cpaL , whose deletion led to reduced piliation levels, and surprisingly, a threefold increase in surface adhesion due to increased holdfast production. To understand this finding, we compared holdfast synthesis in wild-type and cpaL mutant cells under conditions that block pilus retraction. While this treatment increased holdfast production in wild-type cells by triggering the surface-sensing pathway, no increase was observed in the cpaL mutant, suggesting that mutation of cpaL maximally stimulates surface-sensing. Furthermore, when the cpaL mutant was grown in a medium that blocks the surface sensing pathway, cells exhibited decreased surface attachment and holdfast production, consistent with a role for CpaL in pilus-dependent surface sensing in C. crescentus . To better understand the function of CpaL, we analyzed its predicted structure, which suggested that CpaL is a minor pilin fused to a mechanosensitive von Willebrand factor type A (vWA) domain that could be accommodated at the pilus tip. These results collectively position CpaL as a strong candidate for a mechanosensory element in pilus-mediated surface sensing.