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Abstract:
Many gram-negative pathogens utilize a protein complex, termed the type
III secretion system (T3SS), to inject virulence factors from their
cytoplasm directly into the host cell. An export apparatus that is
formed by five putative integral membrane proteins (SctR/S/T/U/V),
resides at the center of the T3SS complex. In this study, we
characterized the smallest export apparatus protein, SctS, which
contains two putative transmembrane domains (PTMD) that dynamically
extract from the inner membrane and adopt a helix-turn-helix structure
upon assembly of the T3SS. Replacement of each SctS PTMD with an
alternative hydrophobic sequence resulted in abolishment of the T3SS
activity, yet SctS self- and hetero-interactions as well as the overall
assembly of the T3SS complex were unaffected. Our findings suggest that
SctS PTMDs are not crucial for the interactions or the assembly of the
T3SS base complex but rather that they are involved in adjusting the
orientation of the export apparatus relative to additional T3SS
sub-structures, such as the cytoplasmic- and the inner-membrane rings.
This ensures the fittings between the dynamic and static components of
the T3SS and supports the functionality of the T3SS complex.
