PMID: 21917917

 

    Legend: Sugar

Title : Membrane topological structure of neutral system N/A amino acid transporter 4 4 (SNAT4 ) protein

Abstract :

1. Members of system N/A amino acid transporter ( SNAT ) family mediate transport of neutral amino acids, including l- alanine , l- glutamine , and l- histidine , across the plasma membrane and are involved in a variety of cellular functions
2. By using chemical labeling, glycosylation, immunofluorescence combined with molecular modeling approaches, we resolved the membrane topological structure of SNAT4 , a transporter expressed predominantly in liver
3. To analyze the orientation using the chemical labeling and biotinylation approach, the " Cys-null" mutant of SNAT4 was first generated by mutating all five endogenous cysteine residues. cysteine residues
4. Based on predicted topological structures, a single cysteine residue was introduced individually into all possible nontransmembrane domains of the Cys-null mutant
5. The cells expressing these mutants were labeled with N-biotinylaminoethyl methanethiosulfonate, a membrane-impermeable cysteine-directed reagent
6. We mapped the orientations of N- and C-terminal domains
7. There are three extracellular loop domains , and among them, the second loop domain is the largest that spans from amino acid residue ∼242 to ∼335
8. The orientation of this domain was further confirmed by the identification of two N-glycosylated residues, Asn-260 and Asn-264
9. Together, we showed that SNAT4 contains 10 transmembrane domains with extracellular N and C termini and a large N-glycosylated, extracellular loop domain
10. This is the first report concerning membrane topological structure of mammalian SNAT transporters, which will provide important implications for our understanding of structure-function of the members in this amino acid transporter family