title: Functional characterization of NAT/NCS2 proteins of Aspergillus brasiliensis reveals a genuine xanthine-uric acid transporter and an intrinsically misfolded polypeptide.
abstract: The Nucleobase-Ascorbate Transporter (NAT) family includes members in nearly all domains of life. Functionally characterized NAT transporters from bacteria, fungi, plants and mammals are ion-coupled symporters specific for the uptake of purines, pyrimidines and related analogues. The characterized mammalian NATs are specific for the uptake of L-ascorbic acid. In this work we identify in silico a group of fungal putative transporters, named UapD-like proteins, which represent a novel NAT subfamily. To understand the function and specificity of UapD proteins, we cloned and functionally characterized the two Aspergillus brasiliensis NAT members (named AbUapC and AbUapD) by heterologous expression in Aspergillus nidulans. AbUapC represents canonical NATs (UapC or UapA), while AbUapD represents the new subfamily. AbUapC is a high-affinity, high-capacity, H(+)/xanthine-uric acid transporter, which can also recognize other purines with very low affinity. No apparent transport function could be detected for AbUapD. GFP-tagging showed that, unlike AbUapC which is localized in the plasma membrane, AbUapD is ER-retained and degraded in the vacuoles, a characteristic of misfolded proteins. Chimeric UapA/AbUapD molecules are also turned-over in the vacuole, suggesting that UapD includes intrinsic peptidic sequences leading to misfolding. The possible evolutionary implication of such conserved, but inactive proteins is discussed.
authors: Faculty of Biology, University of Athens, Panepistimioupolis, Athens 15784, Greece.
authors: Emilia
authors: E
authors: Krypotou
authors: Department of Microbiology, Imperial College, London SW7 2AZ, United Kingdom; Institut de Génétique et Microbiologie, Université Paris-Sud, France.
authors: Claudio
authors: C
authors: Scazzocchio
authors: Faculty of Biology, University of Athens, Panepistimioupolis, Athens 15784, Greece. Electronic address: diallina@biol.uoa.gr.
authors: George
authors: G
authors: Diallinas
mesh_terms: D000595:Amino Acid Sequence; D001230:Aspergillus / Q000235:genetics*; D001233:Aspergillus nidulans / Q000235:genetics / Q000378:metabolism; D001692:Biological Transport; D003198:Computer Simulation; D005656:Fungal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D033702:Nucleobase Transport Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D010455:Peptides / Q000737:chemistry; D010802:Phylogeny; D017510:Protein Folding; D011993:Recombinant Fusion Proteins / Q000138:chemical synthesis / Q000378:metabolism; D016415:Sequence Alignment; D014527:Uric Acid / Q000378:metabolism*; D014970:Xanthines / Q000378:metabolism*
chemical_list: D005656:Fungal Proteins; D033702:Nucleobase Transport Proteins; D010455:Peptides; D011993:Recombinant Fusion Proteins; D014970:Xanthines; D014527:Uric Acid
keywords: Fungi; Phylogeny; Pseudogene; Purine; Specificity