InterPro: IPR003708 Bacterial protein export chaperone SecB

Secretion across the inner membrane in some Gram-negative bacteria occurs via the preprotein translocase pathway. Proteins are produced in the cytoplasm as precursors, and require a chaperone subunit to direct them to the translocase component [1]. From there, the mature proteins are either targeted to the outer membrane, or remain as periplasmic proteins. The translocase protein subunits are encoded on the bacterial chromosome.

The translocase itself comprises 7 proteins, including a chaperone protein (SecB), an ATPase (SecA), an integral membrane complex (SecCY, SecE and SecG), and two additional membrane proteins that promote the release of the mature peptide into the periplasm (SecD and SecF) [1]. The chaperone protein SecB [2] is a highly acidic homotetrameric protein that exists as a "dimer of dimers" in the bacterial cytoplasm. SecB maintains preproteins in an unfolded state after translation, and targets these to the peripheral membrane protein ATPase SecA for secretion [3].

Recently, the tertiary structure of Haemophilus influenzae SecB (P44853) was resolved by means of X-ray crystallography to 2.5A [4]. The chaperone comprises four chains, forming a tetramer, each chain of which has a simple alpha+beta fold arrangement. While one binding site on the homotetramer recognises unfolded polypeptides by hydrophobic interactions, the second binds to SecA through the latter's C-terminal 22 residues.