The family of cold shock proteins of Bacillus subtilis - Stability and dynamics in vitro and in vivo

Bacillus subtilis possesses three homologous small cold shock proteins (CSP s; CspB, CspC, CspD, sequence identity >72%). They share a similar beta-she et structure, as shown by circular dichroism, and have a very low conformat ional stability, with CspC being the least stable. Similar to CspB, CspC an d CspD unfold and refold extremely fast in a N reversible arrow U two-state reaction with average lifetimes of only 100-150 ms for the native state an d 1-6 ms for the unfolded states at 25 degrees C. As a consequence of their low stability and low kinetic protection against unfolding, all three cold shock proteins are rapidly degraded by proteases in vitro, Analysis of the CSP stabilities in vivo by pulse-chase experiments revealed that CspB and CspD are stable during logarithmic growth at 37 degrees C as well as after cold shock. The cellular half-life of CspC is shortened at 37 degrees C, bu t under cold shock conditions CspC becomes stable. The proteolytic suscepti bility of the CSPs in vitro was strongly reduced in the presence of a nucle ic acid ligand, suggesting that the observed stabilization of CSPs in vivo is mediated by binding to their substrate mRNA at 37 degrees C and, in part icular, under cold shock conditions.