Chimeric purine transporters of Aspergillus nidulans define a domain critical for function and specificity conserved in bacterial, plant and metazoan homologues

DIALLINAS, G; VALDEZ, J; SOPHIANOPOULOU, V; ROSA AL; SCAZZOCCHIO C

EMBO JOURNAL

NATURE PUBLISHING GROUP

Año: 1998 vol. 17 p. 3827 - 3837

0261-4189

In Aspergillus nidulans, purine uptake is mediated bythree transporter proteins: UapA, UapC and AzgA.UapA and UapC have partially overlapping functions,are 62% identical and have nearly identical predictedtopologies. Their structural similarity is associated withoverlapping substrate specificities; UapA is a highaffinity,high-capacity specific xanthine/uric acid transporter.UapC is a low/moderate-capacity general purinetransporter. We constructed and characterized UapA/UapC, UapC/UapA and UapA/UapC/UapA chimericproteins and UapA point mutations. The region includingresidues 378–446 in UapA (336–404 in UapC) hasbeen shown to be critical for purine recognition andtransport. Within this region, we identified: (i) oneamino acid residue (A404) important for transporterfunction but probably not for specificity and tworesidues (E412 and R414) important for UapA functionand specificity; and (ii) a sequence, (F/Y/S)X(Q/E/P)NXGXXXXT(K/R/G), which is highly conserved in allhomologues of nucleobase transporters from bacteriato man. The UapC/UapA series of chimeras behavesin a linear pattern and leads to an univocal assignmentof functional domains while the analysis of the reciprocaland ‘sandwich’ chimeras revealed unexpectedinter-domain interactions. cDNAs coding for transportersincluding the specificity region defined by thesestudies have been identified for the first time in thehuman and Caenorhabditis elegans databases.