Lipopolymers, Isoprenoids, and the Assembly of the Gram-Positive Cell Wall

VII. SUMMARYCertain Gram-positive bacterial cell surface polymers may be synthesized by stepwise addition of polymer subunits to an amphipathic acceptor. The presence of a lipid moiety presumably promotes the association of the acceptor with the membrane, enables two-dimensional diffusion in the membrane monolayer, and facilitates export of intermediates through the cellular permeability barrier. In the case of the membrane-bound lipopolymers lipomannan and LTA, the finished product remains covalently linked to an acylated anchor lipid that is associated with the extracellular leaflet of the plasma membrane. In the case of more transient lipopolymers that serve as intermediates in cell wall biosynthesis, such as peptidoglycan and teichoic acid intermediates, two fates are theoretically possible: (1) transfer into wall with concomitant or later cleavage of the lipid moiety, and recycling or secretion of the lipid into the growth medium and (2) transfer into wall without cleavage of the lipid moiety, resulting in maintainance of the covalent bond between lipid moiety and polymer chain. In the latter case, a stable relationship between the cell wall and the plasma membrane would be promoted. A physical relationship at the membrane-wall junction appears to be required for the incorporation of nascent peptidoglycan and teichoic acid into preexisting wall. A number of Gram-positive bacteria have been shown to resist plasmolysis. It is proposed that the existence of a membrane-wall junction might, in part, be the consequence of participation of lipopolymeric intermediates in the assembly of the cell wall.