Export of proteins is necessary for a variety of essential bacterial functions including expression of virulence factors on the cell surface, release of effector proteins to the extracellular milieu, nutrient acquisition, and organelle biogenesis. To export a protein, Helicobacter pylori as a gram-negative bacterium faces the triple barrier of transporting the polypeptide first across the inner membrane (IM), then through the periplasmic space, and finally across the outer membrane (OM). The task of passing through the periplasm is not trivial since proteins may fold and form disulfide bonds in this space before reaching and crossing the OM.
A number of different pathways have been characterized for protein export in gram-negative bacteria. The general secretory pathway (GSP) exports proteins carrying an amino-terminal signal sequence in a stepwise manner, across the IM first, and then across the OM. Proteins secreted via the different terminal branches of the GSP require the Sec system to cross the IM but use different approaches to get through the OM. The Sec-independent pathways are able to transfer proteins directly from the cytoplasm to the outside of the bacteria. In addition, Sec-independent pathways that use other strategies to cross both membranes are known (Table 1). This chapter describes what is known about protein export in H. pylori from the perspective of the general mechanisms for protein export in gram-negative bacteria.
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