Jonathan Reizer

Homology has been established for members of two families of functionally related bacterial membrane proteins. The first family (the resistance/nodulation/cell division (RND) family) includes (i) two metal-resistance efflux pumps in Alcaligenes eutrophus (CzcA and CnrA), (ii) three proteins which function together in nodulation of alfalfa roots by Rhizobium meliloti (NoIGHI), and (iii) a cell division protein in Escherichia coli (EnvD). The second family (the putative membrane fusion protein (MFP) family) includes a nodulation protein (NoIF), a cell division protein (EnvC), and a multidrug resistance transport protein (EmrA). We propose that an MFP functions co-operatively with an RND protein to transport large or hydrophobic molecules across the two membranes of the Gram-negative bacterial cell envelope.

The phosphoenolpyruvate:sugar phosphotransferase system (PTS) consists of two general energy-coupling proteins, enzyme I and HPr, as well as the sugar-specific permeases, commonly referred to as the enzyme II complex (15,19,24,26).The system catalyzes the concomitant transport and phosphorylation of its sugar substrates in a process termed group translocation (16, 25).The PTS permeases found in a variety of bacteria may consist of one, two, three, or four distinct polypeptide chains and, in certain instances, some of these sugar-specific proteins are fused to protein domains which serve the general energy-coupling function(s) of en- zyme I and/or HPr (8,31,34) (see Fig. 1 and 2 for schematic depictions).Evidence suggesting that the entire enzyme II complex is required for concomitant sugar transport and phosphorylation but that enzyme I and HPr merely function to phosphorylate this complex has been presented (4, 9).The enzyme II complex should therefore be considered the functional unit designated permease in this paper.Recent sequence comparison studies have shown that most of the PTS permeases are homologous, i.e., derived from a common ancestral protein (13,21,28,33).These group translocators, comprising the major group of the enzyme II complexes, generally have similar molecular weights of about 68,000, corresponding to about 635 total amino acyl residues (31).Most of them consist either of a single polypeptide chain, commonly termed enzyme II, or of two proteins, commonly called an enzyme II-III pair or an enzyme IIB-IIA pair (15, 31).In a few cases, these perme- ases consist of three or four proteins (6,14,22).Regardless of the number of polypeptide chains in each of these perme- ases, there are always at least three well-recognized func- tional domains: a hydrophobic transmembrane domain which binds and transports the sugar substrate (10), a hydrophilic enzyme III-like domain which possesses the first phosphorylation site (always a histidyl residue [31]), and a second hydrophilic protein or protein domain which pos- sesses the second phosphorylation site (either a cysteyl residue, as for the major group of homologous PTS perme- ases [1,18,22] [see Fig. 3], or a histidyl residue, as for a splinter group of three distinct sequenced PTS permeases which catalyze the transport and phosphorylation of mannose in Escherichia coli, fructose [to yield fructose 6-phos- phate] in Bacillus subtilis, and sorbose in Klebsiella pneu- moniae [5, 12a, 14]).These three permeases of the splinter group lack sufficient sequence similarity with the major group of PTS permeases to establish homology with them, but they are clearly homologous to each other.At the present time, well over 12 PTS permeases have been completely sequenced.While the permeases within the