Mammalian and bacterial sugar transport proteins are homologous

Abstract

The uptake of a sugar across the boundary membrane is a primary event in the nutrition of most cells, but the hydrophobic nature of the transport proteins involved makes them difficult to characterize. Their amino-acid sequences can, however, be determined by cloning and sequencing the corresponding gene (or complementary DNA). We have determined the sequences of the arabinose-H+ and xylose-H+ membrane transport proteins of Escherichia coli. They are homologous with each other and, unexpectedly, with the glucose transporters of human hepatoma and rat brain cells. All four proteins share similarities with the E. coli citrate transporter. Comparisons of their sequences and hydropathic profiles yield insights into their structure, functionally important residues and possible evolutionary relationships. There is little apparent homology with the lactose-H+ (LacY) or melibiose-Na+ (MelB) transport proteins of E. coli.

Secondary solute transport in bacteria

“…These results and those on the homologous sequences in interhelix loops 2-3 and 8-9 of TetA indicate that the conserved R/KXGRR/K (or GXXXXR/ KXGRR/K) motifs [16,126,127,130] are structurally and to some extent functionally important in the secondary carrier proteins. Interhelix loop 9-10 of LacY corresponds with a periplasmic region between ahelices IX-X which may be close to some 'translocation-site' residues (see Section VII).…”

“…In addition, various highly conserved sequence motifs have been observed, such as the 'R/K-X-G,R-R/K' or the extended 'G-X-X-X-D-R/K-X-G-R-R/K' motif in cytoplasmic loops of several transport proteins that is predicted to form a /3-turn structure (see below) [16,126,127]. These motifs have been observed in the polypeptides despite the fact that homology searches places several of these transporters in different families.…”

Transmembrane glutamic acid residues play essential roles in the metal‐tetracycline/H+ antiporter of Staphylococcus aureus

“…In Gram-negative bacteria, tetracycline resistance is mostly assigned to the drug etitux systems, which are classified into A-H [11]. They have a conserved sequence motif, G(K/R)XSD(R/K)XGR(R/K), in the first cytoplasmic loop (named loop2-3), which is common not only in bacterial drug export proteins [12,13] but also in the secondary transporters and belongs to a major facilitator family [14,15]. Tet(K) and Tet(L) also contain a derivative of this motif, GKXSDXX(X/G)XK(K/R), in the corresponding loop, indicating that they belong to a major facilitator family.…”

Experimental tests of a homology model for OxlT, the oxalate transporter of Oxalobacter formigenes

“…All members of the MFS share an architectural theme in which a central loop connects two groups of (typically) six transmembrane ␣-helices. Moreover, the superfamily as a whole is characterized by a short motif (GXXXDK͞R) at the cytoplasmic ends of TM2 and TM8 (3,4), suggesting that these two six-helix clusters derived from a common ancestor; indeed, at times one finds a clear sequence homology between the N-and C-terminal domains (4,5).…”