Permease periplasmic

Figure 11.1 Schematic representation of iron uptake mechanisms, (a) The transferrin-mediated pathway in animals involves receptor-mediated endocytosis of diferric transferrin (Tf), release of iron at the lower pH of the endocytic vesicle and recycling of apoTf. (b) The mechanism in H. influenzae involves extraction of iron from Tf at outer membrane receptors and transport to the inner membrane permease system by a periplasmic ferric binding protein (Fbp). From Baker, 1997. Reproduced by permission of Nature Publishing Group.

Likewise, for zinc, bacteria have developed active uptake systems (Hantke, 2001). In many bacteria the high-affinity Zn2+ uptake system uses an ABC transporter of the cluster 9 family, which mostly transports zinc and manganese and is found in nearly all bacterial species. First identified in cyanobacteria and pathogenic streptococci, but also found in E. coli, the system is encoded by three genes ZnuABC and consists of an outer membrane permease ZnuB, a periplasmic-binding protein ZnuA and a cytoplasmic ATPase ZnuC. Low-affinity transporters of the ZIP family, described later in this chapter, such as ZupT, have also been shown to be involved in bacterial zinc uptake. 

Two are hydrophobic intrinsic membrane domains, each with six membrane-spanning helices and two are peripheral membrane ATP binding domains. All four domains may be in a single peptide chain, as in CFTR, or they may be separate smaller proteins as in bacterial periplasmic permeases.431 446... 

Periplasmic permeases. Gram-negative bacteria contain numerous ABC transporters with components located on the periplasmic surfaces of their plasma membranes. Many of these can be dissociated from the surfaces by osmotic shock, i.e., by sudden changes in the osmotic pressure of the medium.451 452 For example, cells of E. coli suspended in 0.5 M sucrose, treated with 10 4 M EDTA for 10 min, and then diluted... 

Kerpolla RE, Shyamala VK, Klebba P, Ferro-Luzzi Ames G (1991) The membrane-bound proteins of periplasmic permeases form a complex. J Biol Chem 266 9857-9865... 

Bishop L, Agbayani R, Ambudkar SV, Maloney PC, Ames GF-L (1989) Reconstitution of a bacterial periplasmic permease in proteoliposomes and demonstration of ATP hydrolysis concomitant with transport. Proc Natl Acad Sci USA 86 6953-6957... 

Wzy-dependent pathway is most widespread in O polysaccharide biosynthesis. Following the assembly of O repeating units, the individual und-PP-linked O units are exported to the site of polymerization at the periplasmic face of the plasma membrane (Figure 2). This process requires a Wzx protein, the O unit transporter (flippase), which is highly hydrophobic with 12 potential transmembrane domains. Although the Wzx proteins share little primary sequence similarity, they do share structural features with bacterial permeases... 

Essential solutes able to permeate the outer membrane are rapidly sequestered by the appropriate periplasmic binding protein. This lowers the effective free solute concentration facilitating further solute diffusion. The binding protein is capable of mediating the rapid transfer of the solute to the appropriate cytoplasmic membrane permease which transports the ligand to the cell interior. 

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