Determination of the essentiality of the eight cysteine residues of the NrtA protein for high-affinity nitrate transport and the generation of a functional cysteine-less transporter.
All eight cysteine residues, C90, C94, C143, C147, C219, C325, C367, and C431, present in transmembrane domains of the Aspergillus nidulans NrtA nitrate transporter protein were altered individually by site-specific mutagenesis. The results indicate that six residues, C90, C147, C219, C325, C367, and C431, are not required for nitrate transport. Although alterations of C94 and C143 are less well tolerated, these residues are not mandatory and their possible role is discussed. A series of constructs, all completely devoid of cysteine residues, was generated to permit future cysteine-scanning mutagenesis. The optimum cysteine-less combination was identified as C90A, C94A, C143A, C147T, C219S, C325S, C367S, and C431S. This mutant combination yielded transformant strains with up to 40% of wild-type nitrate transport activity. Furthermore, the K(m) value and the level of protein expression were found to be similar to those of the wild-type. This cysteine-less vector should allow us to investigate in detail potentially interesting NrtA amino acids (e.g. identified from homology comparisons) which may be involved in transport, by altering these singly to cysteine and studying such residues by thiol chemistry.
Unkles
Shiela E
SE
School of Biology, University of St Andrews, UK.
Rouch
Duncan A
DA
Wang
Ye
Y
Siddiqi
M Yaeesh
MY
Okamoto
Mamoru
M
Stephenson
Raymond M
RM
Kinghorn
James R
JR
Glass
Anthony D M
AD
D027321:Anion Transport Proteins / Q000737:chemistry*; D001233:Aspergillus nidulans / Q000378:metabolism; D001692:Biological Transport; D002352:Carrier Proteins / Q000737:chemistry* / Q000235:genetics; D003545:Cysteine / Q000737:chemistry*; D005656:Fungal Proteins / Q000737:chemistry*; D016297:Mutagenesis, Site-Directed; D009566:Nitrates / Q000378:metabolism
D027321:Anion Transport Proteins; D002352:Carrier Proteins; D005656:Fungal Proteins; D009566:Nitrates; C444100:NrtA protein, Aspergillus nidulans; D003545:Cysteine