Type 1 and 5 secretion systems and associated toxins - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 486

FIGURE 16.1 Model of the type 1 secretion system. Upon recognition of a C-terminal secre-
tion signal on the secreted protein, the inner membrane complex formed by an energy-providing
ABC cassette and MFP protein interacts with the trimeric OMP. A sealed channel assembly span-
ning across the two membranes of the Gram-negative cell envelope is formed, through which the
substrate is secreted. The secreted protein is translocated in a single step, with no periplasmic
intermediate.
The secreted proteins are diverse in function and include toxins, proteases,
lipases, S-layer proteins, hemophores, bacteriocins, and others with as yet
unknown functions ( Delepelaire, 2004 ). Although divergent in sequence, the
majority of proteins secreted by the T1SS have a C-terminal located secretion
signal, which is recognized by the translocation machinery; the exception is
dispersin of EAEC which is secreted to the periplasm by the Sec system before
secretion via the T1SS to the extracellular milieu ( Koronakis et al., 1989; Ghigo
and Wandersman, 1994; Sheikh et al., 2002 ).
The translocating machinery of a T1SS is comprised of three proteins that
span the cell envelope, all of which are essential for secretion ( Letoffe et al.,
1996 ). Two of the translocation proteins span the inner membrane (the ATP-
binding cassette [ABC] and the membrane fusion/adapter protein [MFP]),
while the final member of the translocator is an outer-membrane protein (OMP)
( Figure 16.1 ). The ABC protein possesses a nucleotide-binding domain (NBD)
fused to a transmembrane domain (TMD), and recognizes the C-terminal secre-
tion signal of the substrate molecule; as such this protein is responsible for the
specificity of the secretion machinery for the substrate molecule ( Delepelaire,
2004 ). The MFP protein consists of a short cytoplasmic domain at the N-
terminus, followed by a membrane anchor and a large periplasmic domain