Class II chaperones: indispensable components of the Gram-negative bacterial T3SS as guardians of translocators.

The type III secretion system (T3SS) is a nanomachinery through which Gram-negative bacteria release effector proteins into host cells and induce cytotoxicity. The T3SS is aided by chaperones, a class of proteins that are involved in various functions of the T3SS, including protecting the components from degradation, secretion pilots, premature association, and maintaining stability. T3SS chaperones are classified into 3 classes, Classes I, II, and III, each assisting a different T3SS component. This review focuses on the Class II chaperone, which interacts with the major and minor translocators that form the translocon. The chaperones associate with the translocators through their hydrophobic residues and deliver them to the tip of the needle, where the translocators form a pore in the host cell. Chaperones play a key role in preventing translocators from degrading due to their hydrophobic nature, and their absence has been found to affect T3SS function. Hence, chaperones play an important role in the pathogenicity of Gram-negative bacteria. This review aims to summarize the interactions, structures, and functions of Class II chaperones. This review is intended to provide more insight into Class II chaperones by outlining the features of chaperones in seven Gram-negative organisms.