Forces and Kinetics of the Bacillus subtilis Spore Coat Proteins CotY and CotX Binding to CotE Inspected by Single Molecule Force Spectroscopy.

Spores are uniquely stable cell types that are produced when bacteria encounter nutrient limitations. Spores are encased in a complex multilayered coat, which provides protection against environmental insults. The spore coat of Bacillus subtilis is composed of around 70 individual proteins that are organized into four distinct layers. Here we explored how morphogenetic protein CotE guides formation of the outermost layer of the coat, the crust, around the forespore by focusing on three proteins: CotE, CotY, and CotX. Single molecule force spectroscopy (SMFS) was used to investigate the interactions among CotE, CotY, and CotX at the single-molecule level. Direct interactions among these three proteins were observed. Additionally, the dissociation kinetics was also studied by measuring the unbinding forces of the complexes at different loading rates. A series of kinetic data of these complexes were acquired. It was found that the interaction of CotE and CotY was stronger than that of CotE and CotX.