Abstract
The cell envelope of parietal monoderm bacteria (archetypal Gram-positive bacteria) is formed of a cytoplasmic membrane (CM) and a cell wall (CW). While the CM is composed of phospholipids, the CW is composed at least of peptidoglycan (PG) covalently linked to other biopolymers, such as teichoic acids, polysaccharides, and/or polyglutamate. Considering the CW is a porous structure with low selective permeability contrary to the CM, the bacterial cell surface hugs the molecular figure of the CW components as a well of the external side of the CM. While the surfaceome corresponds to the totality of the molecules found at the bacterial cell surface, the proteinaceous complement of the surfaceome is the proteosurfaceome. Once translocated across the CM, secreted proteins can either be released in the extracellular milieu or exposed at the cell surface by associating to the CM or the CW. Following the gene ontology (GO) for cellular components, cell-surface proteins at the CM can either be integral (GO: 0031226), i.e., the integral membrane proteins, or anchored to the membrane (GO: 0046658), i.e., the lipoproteins. At the CW (GO: 0009275), cell-surface proteins can be covalently bound, i.e., the LPXTG-proteins, or bound through weak interactions to the PG or wall polysaccharides, i.e., the cell wall binding proteins. Besides monopolypeptides, some proteins can associate to each other to form supramolecular protein structures of high molecular weight, namely the S-layer, pili, flagella, and cellulosomes. After reviewing the cell envelope components and the different molecular mechanisms involved in protein attachment to the cell envelope, perspectives in investigating the proteosurfaceome in parietal monoderm bacteria are further discussed.
Highlights
As the interface of the cell with its surrounding, the bacterial cell surface plays a crucial role in all types of interactions
The cell envelope of parietal monoderm bacteria is composed of a cytoplasmic membrane (CM) and a cell wall (CW), which can be divided into the inner wall zone (IWZ) and outer wall zone (OWZ) (Merchante et al, 1995; Matias and Beveridge, 2005; Zuber et al, 2006; Figure 1)
The cellulosome is generally exposed at the bacterial cell surface following cohesin-dockerin interaction with cell-surface proteins, themselves anchored in the CW via S-layer homology domain (SLH) domains
Summary
As the interface of the cell with its surrounding, the bacterial cell surface plays a crucial role in all types of interactions. Some organelles can be present and result from the assembly of protein subunits to form supramolecular structures, such as the well-known pili and FIGURE 1 | The surfaceome of parietal monoderm bacteria with respect of the organisation and composition of the cell envelope. As revealed by electron microscopy studies and contrary to the OWZ, the IWZ is a thinner zone of low density most certainly devoid of most cross-linked polymeric CW network, except LTAs and some proteins, e.g., lipoproteins (Matias and Beveridge, 2005, 2006); because this zone is not strictly bounded by two biological membranes like in diderm bacteria, the IWZ resembles but cannot be considered as a periplasm sensu stricto, i.e., it presents some analogies but no homology (Buist et al, 2008; Chagnot et al, 2013).
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