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 . 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) . The chaperone
protein SecB  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 .
Recently, the tertiary structure of Haemophilus influenzae SecB was resolved
by means of X-ray crystallography to 2.5A . 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.