Abstract
The cell wall peptidoglycan of Gram-positive bacteria functions as a surface organelle for the transport and assembly of proteins that interact with the environment, in particular, the tissues of an infected host. Signal peptide-bearing precursor proteins are secreted across the plasma membrane of Gram-positive bacteria. Some precursors carry C-terminal sorting signals with unique sequence motifs that are cleaved by sortase enzymes and linked to the cell wall peptidoglycan of vegetative forms or spores. The sorting signals of pilin precursors are cleaved by pilus-specific sortases, which generate covalent bonds between proteins leading to the assembly of fimbrial structures. Other precursors harbour surface (S)-layer homology domains (SLH), which fold into a three-pronged spindle structure and bind secondary cell wall polysaccharides, thereby associating with the surface of specific Gram-positive microbes. Type VII secretion is a non-canonical secretion pathway for WXG100 family proteins in mycobacteria. Gram-positive bacteria also secrete WXG100 proteins and carry unique genes that either contribute to discrete steps in secretion or represent distinctive substrates for protein transport reactions.
Highlights
Hans Christian Gram used light microscopy to detect microbes that were stained with crystal violet/iodine [1]
The differential staining property is based on the peptidoglycan layer, which is considerably thicker in Gram-positive microbes [2]
Another difference is that Gram-positive bacteria elaborate a single membrane, whereas Gram-negative microbes harbour a plasma membrane and an additional outer membrane with lipopolysaccharides [3,4]
Summary
Hans Christian Gram used light microscopy to detect microbes that were stained with crystal violet/iodine [1]. The differential staining property is based on the peptidoglycan layer, which is considerably thicker in Gram-positive microbes [2]. Another difference is that Gram-positive bacteria elaborate a single membrane, whereas Gram-negative microbes harbour a plasma membrane and an additional outer membrane with lipopolysaccharides [3,4]. The secretion of signal peptide-bearing precursor proteins in Gram-positive microbes leads by default to their release into extracellular medium [5]. This review summarizes briefly what is known about the secretion or assembly of proteins in the envelope of Gram-positive microbes. Proteins in any one of the aforementioned locations fulfil unique physiological roles that aid bacteria in their interaction with the environment, most notably the tissues and immune cells of an infected host [23] (figure 1)
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