structural characterization of bacterial secretion channels

National Institute Of Diabetes And Digestive And Kidney Diseases

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Abstract

Gram-negative bacteria, mitochondria, and chloroplasts contain an inner and outer membrane. The outer membrane contains a host of beta-barrel proteins commonly called outer membrane proteins (OMPs), which serve essential functions in cargo transport and signaling and are also vital for membrane biogenesis. In Gram-negative bacteria, it is known that OMPs are synthesized in the cytoplasm and then transported across the inner membrane into the periplasm via a Sec translocon. Once in the periplasm, chaperones guide the nascent OMPs across the periplasm and peptidoglycan to the inner surface of the outer membrane. Here, the nascent OMPs are recognized by a complex known as the beta-barrel assembly machinery (BAM) complex which folds and inserts the new OMPs into the outer membrane. Similar mechanisms for OMP biogenesis exist for mitochondria and chloroplasts, where interaction of the bnacterial or mitochondrial signal sequence located in the final transmembrane beta strand associates with the first beta strand of BamA/Sam50, followed by strand integration into the BAM/SAM complex. We solved structures of BamA (2013), the BAM complex (2016), and the SAM complex (2021). However, even with these structures being known, the mechanism for how the BAM or SAM complex recognizes, folds, and inserts nascent OMPs into the outer membrane remains to be determined. We wrote a comprehensive review on this topic in 2021, while in 2022, we published a review on fungal outer membrane protein biogenesis in Current Opinion in Structural Biology (2022). Ongoing research 2023: Current experiments on the SAM complex probe the binding of mitochondrial beta signals to Sam50, as well as studying binding of a novel antibiotic that has been shown to bind to BamA and inhibit strand integration. This manuscript is in preparation and will be published in 2024. References: Noniaj, N., Kuszak, A.J., Gumbart, J.C., Lukacik, P., Chang, H., Easley, N.C., Lithgow, T. & Buchanan, S.K. (2013). Structural insight into the biogenesis of beta barrel membrane proteins. Nature 501: 385-390. PMCID:PMC3779476 Noinaj, N., Kuszak, A.J., Balusek, C. Gumbart, J.C. & Buchanan, S.K. (2014). Lateral opening and exit pore formation are required for BamA function. Structure 22:1055-62. PMCID: PMC4100585 Bakelar, J., Buchanan, S.K. & Noinaj, N. (2016). The structure of the -barrel assembly machinery complex. Science 351:180-186. PMCID: PMC4883095 Noinaj N, Gumbart JC, Buchanan SK (2017) The -barrel assembly machinery in motion. Nat Rev Microbiol 15:197-204 PMCID: PMC5455337 Diederichs K.A., Ni X., Rollauer S.E., Botos I., Tan X., King M.S., Kunji E.R.S., Jiang J., Buchanan S.K. (2020). Structural insight into mitochondrial -barrel outer membrane protein biogenesis. Nat Commun. 2020 Jul 3;11(1):3290. doi: 10.1038/s41467-020-17144-1.PMID: 32620929 Free PMC article. Diederichs, K.A., Buchanan, S.K. & Botos, I. (2121) Building better barrels - -barrel biogenesis and insertion in bacteria and mitochondria. J Mol Biol 433(16):166894. PMCID: PMC8292188 Varughese, J. Buchanan, S.K. & Pitt, A.S. (2021). The role of Voltage-Dependent Anion Channel in mitochondrial dysfunction and human disease. Cells. 2021 Jul 9;10(7):1737. doi: 10.3390/cells10071737. PMCID: PMC8305817 Pitt, A.S. & Buchanan, S.K. (2021). A biochemical and structural understanding of TOM complex interactions and implications for human health and disease. Cells. 2021 May 11;10(5):1164. doi: 10.3390/cells10051164. PMCID: PMC8150904 Guerin, J. & Buchanan, S.K. (2021). Protein import and export across the bacterial outer membrane. Curr Opin Struct Biol, 69:55-62.doi: 10.1016/j.sbi.2021.03.007. PMCID: PMC8405454 Diederichs, K.A., Pitt, A.S., Varughese, J.T., Hackel, T.N. Buchanan, S.K. & Shaw, P.L. (2022). Mechanistic insights into fungal mitochondrial outer membrane protein biogenesis. Curr Opin Struct Biol, 2022 Jun;74:102383. doi: 10.1016/j.sbi.2022.102383. Epub 2022 Apr 30. PMCID: PMC9189057