Cell Wall-Anchored Surface Proteins and Lipoproteins of Gram-Positive Bacteria

Abstract

The cell wall envelope of gram-positive bacteria is composed of the peptidoglycan macromolecule that functions to protect microbes from osmotic lysis. Small molecules can diffuse through the peptidoglycan layer; however, cellular uptake requires dedicated mechanisms for transport across the cytoplasmic membrane. These and other functions in gram-positive bacteria require lipoproteins, polypeptides that are tethered to the cytoplasmic membrane by an N-terminal lipid modification. This chapter summarizes what is known about the cell wall-anchored surface proteins as well as the targeting mechanism of lipoproteins. Furthermore, some of the many biological functions that these polypeptides fulfill are described. Cell wall sorting signals from surface proteins of other gram-positive bacteria display similar functions. Gram-positive bacteria cannot recycle degraded peptidoglycan fragments and release these compounds into the extracellular medium. A database search using the lipobox of lipoproteins revealed 114 lipoprotein genes in the genome of Bacillus subtilis as compared with 89 lipoproteins encoded in the Escherichia coli genome. Several proteins that are periplasmic carbohydrate binding proteins in gram-negative bacteria were observed to be lipid-modified in gram-positive bacteria. These lipoproteins function to capture specific import substrates and deliver them to the membrane-embedded transport machinery. Several lipoproteins are required for sporulation or geimination of B. subtilis, while the B. subtilis PrsA lipoprotein is a peptidyl-prolyl cis/trans isomerase that assists the folding of secreted polypeptides. Other lipoproteins are thought to be involved in DNA binding or uptake, oxidative phosphorylation, cell wall biogenesis, and autolysis.