Other components of grampositive cell envelopes

Gram-positive bacteria lack an outer membrane but proteins, lipoproteins and other macromolecules are associated with the cytoplasmic membrane and pep-tidoglycan. Many wall proteins are anchored directly to the peptidoglycan, and contain a conserved amino acid motif - LPXTG - which is involved in transporting these proteins across the cytoplasmic mem brane. Lipoproteins, which may be highly immunogenic, are anchored to the cytoplasmic membrane by the N-terminus which is lipid modified. Expression of both wall and cytoplasmic membrane proteins may be influenced by environmental factors such as iron availability. Also associated with the surface of some gram-positive bacteria are immunglobulin-binding proteins such as staphylococcal protein A and streptococcal protein G, which bind the Fc region of antibody molecules. Proteins with analogous function are now being detected in some gram-negative bacteria. Release of such proteins may protect the organism by binding to antibody and preventing its deposition on the bacterial surface. Other proteins such as the M protein of Streptococcus pyogenes have an antiphagocytic function.

Teichoic and lipoteichoic acids are acidic polymers of glycerol or ribitol and are bound to the peptidoglycan and cytoplasmic membrane respectively. They may be important antigens in staphylococci and streptococci, stimulating both humoral and cell-mediated immune responses.

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