DERIVED STRUCTURE OF THE PUTATIVE SIALIC-ACID TRANSPORTER FROM ESCHERICHIA-COLI PREDICTS A NOVEL SUGAR PERMEASE DOMAIN

Catabolism of sialic acids by Escherichia coli requires the genes nanA and nanT, which were previously mapped between argG and rpoN (E. R. Vimr and F. A. Troy, J. Bacteriol. 164:845-853, 1985). This organization is confirmed and extended by physical mapping techniques. An open reading frame beginning 135 bp from the nanA translational stop codon could code for a 53,547-Da hydrophobic polypeptide predicted to contain 14 transmembrane segments. Complementation analysis confirmed that nanT is required for sialic acid uptake when expressed in trans. NanT is homologous to a putative permease encoded by open reading frame 425, which maps between leuX and fecE in the E. coli chromosome. However, unlike this hypothetical permease or previously reported monosaccharide transporters, NanT contains a centrally located domain with two additional potential membrane-spanning segments plus one amphiphilic alpha-helix that may be important for the structure and function of sialic acid-permease.