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  • Heterocysts provide the required microoxic environment for t

    2021-11-29

    Heterocysts provide the required microoxic environment for the oxygen-sensitive N2-fixing nitrogenase enzyme complex. This environment is formed during heterocyst development. Developing heterocysts inactivate photosystem II and degrade antenna pigments, thereby decreasing oxygenic photosynthesis (Fig. 1). In specialized honeycomb membranes at the poles of each heterocyst, oxygen-consuming respiration is enhanced. Heterocysts also build an additional envelope around the outer membrane comprised of two layers: an external heterocyst exopolysaccharide (hep) layer and an underlying heterocyst glycolipid layer (hgl). The hgl layer is made of heterocyst-specific glycolipids (HGLs) and is gas-tight to restrict oxygen infiltration into the heterocyst; the hep layer provides mechanical support for the hgl layer [reviewed in (Adams and Duggan, 1999; Herrero et al., 2016; Kumar et al., 2010; Maldener et al., 2014; Muro-Pastor and Hess, 2012; Nicolaisen et al., 2009; Wolk et al., 1994)]. The complexity of the lifestyle of Anabaena sp., with the ability to perform oxygenic photosynthesis, differentiate specialized cells, and fix N2, demands a sophisticated cell envelope with various transporters for the import and export of different molecules. A search of the Cyanobase genome server (http://genome.microbedb.jp/CyanoBase) reveals around 200 genes in the genome of Anabaena sp. that are annotated as being transporter associated; the vast majority belong to the superfamily of ATP-binding cassette (ABC) transporters (Shvarev and Maldener, 2018). This superfamily is one of the largest protein cediranib australia and has been extensively reviewed (Davidson et al., 2008; Greene et al., 2018; Higgins, 2001, 1992; Holland, 2011; Locher, 2016; Trowitzsch and Tampé, 2018; Wilkens, 2015). Most ABC transporters are homodimers, with each monomer consisting of a cytoplasmic nucleotide-binding domain (NBD) and a transmembrane domain (TMD). These domains can be located on one protein or on two separate proteins. TMDs form the route for substrate transport through the cell membrane; the conformational changes necessary for transport are enabled by ATP hydrolysis in the NBD. In bacteria, both ABC importers and exporters exist. They fulfill various functions, e.g., nutrient uptake, secretion of cell envelope components, and drug resistance. Some ABC exporters of gram-negative bacteria interact with the outer membrane exit duct, such as the Escherichia coli outer membrane protein TolC, via specific periplasmic adaptor proteins, known as membrane fusion proteins (MFP), thereby forming tripartite efflux pumps (Du et al., 2018). The homotrimer TolC forms a β-barrel pore in the outer membrane and an α-barrel in the periplasm and interacts with inner membrane transporters via various MFPs. These tripartite efflux pumps provide a channel for export of various substrates across the gram-negative cell envelope, e.g., drugs and proteinaceous toxins (Du et al., 2018; Greene et al., 2018; Hinchliffe et al., 2013; Koronakis et al., 2004; Wandersman and Delepelaire, 1990). In this minireview, we will discuss the existing knowledge about a specific group of ABC transporters, namely the DevBCA-HgdD-like tripartite efflux pumps, and their roles in the physiology of Anabaena sp.
    Conclusions The complex physiology and multicellular lifestyle of Anabaena sp. PCC 7120 requires a number of sophisticated transport machineries of different types, with a prevalence of ABC transporters. ABC-transporter-driven tripartite efflux pumps play crucial roles in the Anabaena sp. life cycle. These pumps are essential for heterocyst development, antibiotic resistance, and probably other aspects of growth (Fig. 3). Despite their very similar predicted topology, their functions and substrates differ and are specific. Further investigation of these efflux pumps will elucidate unknown details of the functions and biology of ABC transporters. Such a basic understanding is of great importance considering the various clinical problems related to ABC transporters, such as the multiple drug resistance of pathogenic bacteria and cancer cells.