SecB

SecB is a cytosolic molecular chaperone in many bacteria, best characterized in Escherichia coli, where it functions in the Sec-dependent protein export pathway. It binds unfolded or partially folded secretory preproteins in the cytoplasm, preventing aggregation and maintaining a translocation-competent state. SecB then delivers these substrates to the SecA/SecYEG translocon at the inner membrane, promoting post-translational targeting of proteins destined for the periplasm or outer membrane. The interaction with SecA facilitates substrate handoff and translocation through the Sec channel; SecB itself does not hydrolyze ATP.

Structurally, SecB operates as a homotetramer; each subunit is roughly 150 amino acids with a total molecular mass around 64 kDa for the tetramer. The tetrameric assembly provides multiple substrate-binding surfaces, enabling simultaneous engagement of different preproteins.

Substrates recognized by SecB include a broad range of secretory proteins, such as maltose-binding protein (MalE), outer membrane protein OmpA, alkaline phosphatase (PhoA), and others bearing N-terminal signal peptides. While SecB is important for efficient export of many secretory proteins in Gram-negative bacteria, it is not universal or strictly essential; some species lack SecB and rely on alternative chaperone pathways or direct interactions with SecA.

Physiologically, SecB contributes to periplasmic and outer membrane protein biogenesis and helps maintain cytosolic proteostasis by preventing premature folding and aggregation of preproteins. The SecB pathway was established through genetic and biochemical studies in E. coli and remains a paradigm for post-translational targeting in bacteria.