Surface proteins of gram-positive bacteria and mechanisms of their targeting to the cell wall envelope

The cell wall envelope of gram-positive bacteria is a macromolecular, exosk eletal organelle that is assembled and turned over at designated sites. The cell wall also functions as a surface organelle that allows gram-positive pathogens to inter-act with their environment, in particular the tissues of the infected host. All of these functions require that surface proteins an d enzymes be properly targeted to the cell wall envelope. Two basic mechani sms, cell wall sorting and targeting have been identified Cell well sorting is the covalent attachment of surface proteins to the peptidoglycan via a C-terminal sorting signal that contains a consensus LPXTG sequence. More th an 100 proteins that possess cell wall-sorting signals, including the M pro teins of Streptococcus pyogenes, protein A of Staphylococcus aureus and sev eral internalins of Listeria monocytogenes, have been identified Cell wall targeting involves the noncovalent attachment of proteins to the cell surfa ce via specialized binding domains. Several of these wall-binding domains a ppear to interact with secondary wall polymers that are associated with the peptidoglycan, for example teichoic acids and polysaccharides. Proteins th at are targeted to the cell surface include muralytic enzymes such as autol ysins, lysostaphin, and phage lytic enzymes. Other examples for targeted pr oteins are the surface S-layer proteins of bacilli and clostridia, as well as virulence factors required for the pathogenesis of L. monocytogenes (int ernalin B) and Streptococcus pneumoniae (PspA) infections. In this review w e describe the mechanisms for both sorting and targeting of proteins to the envelope of gram-positive bacteria and review the functions of known surfa ce proteins.