Chapter 20 : Alternative Mechanisms of Protein Release

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This chapter reviews recent results concerning alternative strategies that 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 . The second section discusses evidence that 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 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 genes linked to the type IV secretion apparatus. On the basis of biochemical and genetic studies of the secretion apparatus, the locations of the six 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 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), . ASM Press, Washington, DC. doi: 10.1128/9781555818005.ch20

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Type IV Secretion System Proteins
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Figure 1

homologs of the conjugation machine. (A) Genetic organization of the conjugation machine. type IV transporter components with homology to the Vir proteins are aligned underneath their homologs. (B) A model of the conjugation machine with the location of the 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.

Citation: Blanke S, Ye D. 2001. Alternative Mechanisms of Protein Release, p 227-237. In Mobley H, Mendz G, Hazell S (ed), . ASM Press, Washington, DC. doi: 10.1128/9781555818005.ch20
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Image of Figure 2
Figure 2

. altruistic autolysis. This model predicts that responds to specific environmental signals to activate genetically programmed bacterial autolysis. Specific protein autolysins degrade the 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 survival in the acid environment and colonization of the gastric mucosa.

Citation: Blanke S, Ye D. 2001. Alternative Mechanisms of Protein Release, p 227-237. In Mobley H, Mendz G, Hazell S (ed), . ASM Press, Washington, DC. doi: 10.1128/9781555818005.ch20
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Figure 3

outer membrane vesicles budding. The release of outer membrane vesicles by 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).

Citation: Blanke S, Ye D. 2001. Alternative Mechanisms of Protein Release, p 227-237. In Mobley H, Mendz G, Hazell S (ed), . ASM Press, Washington, DC. doi: 10.1128/9781555818005.ch20
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Table 1

type IV system

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

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