In general, it is believed that the entire passenger domains
could be replaced with a target protein, while the N-terminal
signal sequence and the b-domain at the c-terminus are
necessary to mediate the translocation of a recombinant protein
through the inner and outer membranes, respectively. As
such, one conceivable engineering strategy for production of
extracellular proteins from a gram-negative bacterium is to
create a chimera gene by fusing the N-terminal signal
sequence with the target gene encoding the protein to be
transported and the b-domain sequence. However, there are
cases that researchers found that parts or the entire passenger
domains are important to the translocation of a recombinant
protein. For example, Lum et al. investigated the importance
of the passenger domain from the Shigella flexneri autotransporter
in secretion of the TEM-1 b-lactamase and
maltose binding proteins