The B12 receptor BtuB alters the membrane integrity of Caulobacter crescentus

Vitamin B-12 is one of the most complex biomolecules in nature. Since few organisms can synthesize B-12 de novo, most bacteria utilize highly sensitive and specialized transporters to scavenge B-12 and its precursors. In Gram-negative bacteria, BtuB is the outer membrane TonB-dependent receptor for B-12. In the fresh water bacterium Caulobacter crescentus, btuB is among the most highly expressed genes. In this study, we characterized the function of BtuB in C. crescentus and unveiled a potential new function of this receptor involved in cellular fitness. Under standard minimal or rich growth conditions, we found that supplements of vitamin B-12 to cultures of C. crescentus provided no significant advantage in growth rate. Using a B-12 methionine auxotroph, we showed that BtuB in C. crescentus is capable of transporting B-12 at low pico-molar range. A btuB knockout strain displayed higher sensitivity to detergents and to changes in osmotic pressure compared to the wildtype. Electron micrographs of this knockout strain revealed a morphology defect. The sensitivity observed in the btuB knockout strain was not due to changes in membrane permeability or altered S-layer levels. Our results demonstrate that btuB deletion mutants exhibit increased susceptibility to membrane stressors, suggesting a potential role of this receptor in membrane homeostasis. Because we only tested BtuB's function under laboratory conditions, we cannot eliminate the possibility that BtuB also plays a key role as a B-12 scavenger in C. crescentus when growing in its highly variable and nutrient-limited natural environment.