Alternative Mechanisms of Protein Release

Steven R. Blanke; Dan Ye

doi:10.1128/9781555818005.ch20

Chapter 20 : Alternative Mechanisms of Protein Release

Authors: Steven R. Blanke¹, Dan Ye¹

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Affiliations: 1: Department of Biology and Biochemistry, University of Houston, TX 77204

Content Type: Monograph

Publication Year: 2001

Category: Bacterial Pathogenesis

Book DOI: 10.1128/9781555818005

Chapter DOI: 10.1128/9781555818005.ch20

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Abstract:

This chapter reviews recent results concerning alternative strategies that Helicobacter pylori strains employ to introduce proteins into the environment. The chapter is divided into three sections. The first section describes the cellular machinery comprising the type IV secretion system of H. pylori. The second section discusses evidence that H. pylori undergoes autolysis as a mechanism for releasing cytoplasmic proteins directly into the extracellular environment. The third section reviews recent data supporting the theory that cellular envelope proteins are released from H. pylori via the formation of outer membrane vesicles, which appear to have the ability to be taken up into mammalian cells. The chapter focuses on the cag genes linked to the H. pylori type IV secretion apparatus. On the basis of biochemical and genetic studies of the A. tumefaciens secretion apparatus, the locations of the six H. pylori Cag proteins are predicted via analogy to their Vir homologs to be either part of the mating channel, or, alternatively, one of the putative ATPases. The presence of cytoplasmic proteins on the surface of the H. pylori outer membrane both in vitro and in vivo is consistent with a mechanism of bacterial autolysis. In addition, the dependence of cytoplasmic protein release on the phase of bacterial growth in vitro suggests that the autolysis process is regulated. Bacterial autolysis probably involves mechanisms of peptidoglycan degradation to release cytoplasmic contents into the extracellular medium, which can then be strategically relocalized onto the outer membrane of other bacteria.

Citation: Blanke S, Ye D. 2001. Alternative Mechanisms of Protein Release, p 227-237. In Mobley H, Mendz G, Hazell S (ed), Helicobacter pylori. ASM Press, Washington, DC. doi: 10.1128/9781555818005.ch20

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Alternative Mechanisms of Protein Release

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Figure 1

H. pylori homologs of the A. tumefaciens conjugation machine. (A) Genetic organization of the A. tumefaciens conjugation machine. H. pylori type IV transporter components with homology to the Vir proteins are aligned underneath their homologs. (B) A model of the A. tumefaciens conjugation machine with the location of the H. pylori homologs in (A) indicated by arrows with dashed lines. This model is compiled from biochemical data described in the text predicting both the localization and interactions of the individual Vir proteins.

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Figure 1

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Figure 2

H. pylori altruistic autolysis. This model predicts that H. pylori responds to specific environmental signals to activate genetically programmed bacterial autolysis. Specific protein autolysins degrade the H. pylori peptidoglycan layer to lyse the cell envelope and release cytoplasmic proteins such as urease and catalase. Released urease is subsequently adsorbed to the surface of the neighboring bacteria and is important for H. pylori survival in the acid environment and colonization of the gastric mucosa.

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Figure 2

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Figure 3

H. pylori outer membrane vesicles budding. The release of outer membrane vesicles by H. pylori is an alternative mechanism for the delivery of bacterial toxins and antigens to the gastric mucosa. These small vesicles are 50 to 300 nm in diameter. They contain VacA and other proteins such as porins within the trilayered membrane (OM, outer membrane; PS, periplasm; IM, inner membrane; C, cytoplasm).

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Figure 3

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Tables

Alternative Mechanisms of Protein Release

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Table 1

H. pylori type IV system

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10.1128/9781555818005/tab20-1.gif

Table 1

H. pylori type IV system