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Finally, the translocation and assembly module (TAM) was recently
described to be required for efficient secretion of ATs in proteobacteria. The
TAM consists of two proteins, TamA in the outer membrane and TamB in
the inner membrane that together span the entire cell envelope. TamA, like
BamA and TpsB proteins discussed above, belongs to the Omp85 super-
family of outer-membrane proteins. It consists of a C-terminal β-barrel in
the outer membrane and three N-terminal POTRA domains thought to inter-
act with TamB ( Selkrig et al., 2012 ). Mutants lacking the TAM complex in
E.coli failed to secrete the type 5a adhesins Antigen 43 and EhaA ( Selkrig
et al., 2012 ). It is as yet unknown if the TAM complex is required for effi-
cient secretion of type 5b-e proteins. A summary of AT secretion is found in
Figure 16.4 .
FIGURE 16.4 General model of autotransporter secretion. (A) In all proteins the signal sequence
mediates inner-membrane translocation in a Sec-dependent manner. (B) On entering the periplasm
many AT proteins have been shown to interact with periplasmic chaperones which keep the passen-
ger domain in a translocation-competent form. (C) The β-barrel of the AT is inserted into the outer
membrane via interactions with the BAM complex. (D) The translocation domain then mediates
secretion of the passenger domain to the surface. Recently the Tam complex was shown to be critical
for passenger domain secretion for AT proteins. (E) Once on the surface the passenger domain can
either remain attached or be cleaved and secreted.
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