MINIREVIEW Peptidoglycan as a Barrier to Transenvelope Transport

The bacterial envelope can be considered a combined mechanical and permeability barrier. This duality of function protects the cell against detrimental environmental influences and allows the maintenance of a high internal osmotic pressure. Although these properties imply rigidity, the barrier should also be flexible, permitting morphogenetic changes during growth and division and allowing for transport out of and into the bacterial cell. The mechanical integrity and permeability characteristics of the envelope are usually assigned separately to the two major constituents of the envelope: peptidoglycan is considered to be responsible for the mechanical integrity, whereas the membrane(s) constitute(s) the permeability barrier. Although this functional division is for the most part valid, it overlooks the possible contribution of peptidoglycan to the permeability characteristics of the envelope. For gram-positive bacteria, this issue was discussed in some early studies (30, 88), and the fact that the multilayered peptidoglycan, with its high degree of cross-linking and the presence of teichoic acids, might act as a permeability barrier is also the key feature of the recently proposed concept of gram-positive periplasm (68). For gram-negative bacteria, on the other hand, the permeability characteristics of peptidoglycan were overshadowed by those of the outer membrane (72). In fact, whereas low-molecular-weight compounds probably diffuse readily through peptidoglycan, the size of its pores may well be a limiting factor for the passage of larger proteins and protein complexes. In particular, the penetration of peptidoglycan by large multisubunit protein complexes that span the entire envelope may pose considerable steric problems. The presence of these envelopespanning structures has been demonstrated or postulated for a variety of transfer processes such as the secretion of toxins, the assembly of flagella and fimbriae, conjugation, and transformation. However, when the putative structure or function of these large multiprotein transfer machineries is described, the issue of peptidoglycan penetration is not usually taken into consideration. This is illustrated by the failure to depict the peptidoglycan layer in several schematic representations of these large putative envelope-spanning structures. The presence of peptidoglycan pores large enough to accommodate these structures is often taken for granted. On the other hand, some indirect evidence has led to speculation about possible peptidoglycan rearrangements during the assembly of these structures or during the actual transfer process (19, 26, 27, 36, 66). This minireview takes this speculation further by looking at the phenomenon of envelope spanning from a dimensional perspective: we focus on the correlation between the structural and functional features of peptidoglycan and the envelopespanning protein complexes. The observed dimensional discrepancies, combined with an examination of the reported links between the assembly of envelope-spanning complexes and peptidoglycan metabolism, strongly suggest the involvement of specific peptidoglycan-processing enzymes in this assembly. This hypothesis is supported by the recent identification of peptidoglycan hydrolase homologs in several crossenvelope transfer systems.