ABCMdb

PMID: 9511930

Akabas MH, Cheung M, Guinamard R
Probing the structural and functional domains of the CFTR chloride channel.
J Bioenerg Biomembr. 1997 Oct;29(5):453-63., [PubMed]

Sentences

No. Mutations Sentence Comment

81 ABCC7 p.Lys335Cys The effect of modification of the K335C mutant by the MTS reagents on CFTR current would be indirect. Login to comment 87 ABCC7 p.Gln353Cys DIAMETER OF THE CHANNEL LUMEN MTSET+ can penetrate from the extracellular end to react with Q353C (Cheung and Akabas, 1996). Login to comment 89 ABCC7 p.Gln353Cys Therefore, the channel diameter from the extracellular end to the position of Q353C, flanking the cytoplasmic end of the M6 segment, must be at a minimum 6 A. Login to comment 115 ABCC7 p.Leu333Cys ABCC7 p.Leu333Cys The ratio of the reaction rates of MTSES- /MTSET+ with L333C, the most extracellular residue tested, is also 0.08, suggesting that there is no charge selectivity for the movement of the MTS reagents from the extracellular solution to L333C. Login to comment 116 ABCC7 p.Leu333Cys Thus, in order to normalize for the difference in the intrinsic reaction rates of the two MTS reagents we divide the ratio of the rates at a given residue by the ratio of the reaction rates with L333C. Login to comment 122 ABCC7 p.Gln353Cys ABCC7 p.Thr351Cys The major site of charge selectivity appears to be in the region of T351C and Q353C where the anion- to-cation selectivity rises to between 15 and 25 (Fig. 3B) (Cheung and Akabas, 1997). Login to comment 127 ABCC7 p.Gln353Cys ABCC7 p.Thr351Cys Arg352, which is between T351C and Q353C, appears to be a majordeterminantof the anion selectivity in this region. Login to comment 128 ABCC7 p.Arg352Cys Thus, when cysteine is substituted for Arg352 the charge selectivity drops to the same low level that is observed in the more extracellular portion of the channel (Fig. 3B). Login to comment 129 ABCC7 p.Arg352Cys If other residues in this region were the main determinants of anion selectivity, then, the anion selectivity of the R352C mutant should have been similar to that of the adjacent residues. Login to comment 130 ABCC7 p.Arg352Cys ABCC7 p.Gln353Cys ABCC7 p.Thr351Cys Moreover, based on our measurements of electrical distance, R352C is closer to the extracellular end of the channel than is T351C or Q353C (Fig. 3A). Login to comment 131 ABCC7 p.Gln353Cys ABCC7 p.Thr351Cys Thus, ions passing from the extracellular end of the channel would first encounter Arg352, which we infer forms part of the charge-selectivity filter, before they could reach T351C or Q353C, thereby accounting for the greater anion selectivity observed at these residues. Login to comment 140 ABCC7 p.Lys335Glu ABCC7 p.Lys95Asp The mutations K95D and K335E altered the halide permeability sequence, leading to the suggestion that these residues might be involved in anion binding (Anderson et al., 1991b). Login to comment 141 ABCC7 p.Lys335Glu The mutation K335E, however,did not alter the SCN- - induced anomalous mole fraction effects (Tabcharani et al., 1993), and our data imply that Lys335 is on the opposite side of the helix from the channel-liningface (Fig. 2B) (Cheung and Akabas, 1996), making it unlikely that this residue is part of an anion binding site. Login to comment 142 ABCC7 p.Lys335Glu More likely,the K335E mutationinduces aconfor- mational change in residues that form ananion-binding site, possibly at quite a distance from this residue. Login to comment 143 ABCC7 p.Pro99Leu A similar mechanism is likely involved in the alteration of the halide permeability sequence by the P99L mutation (Sheppard et al., 1996) because Pro99 is also not a channel-lining residue (Akabas et al., 1994b). Login to comment 148 ABCC7 p.Leu333Cys ABCC7 p.Ser341Cys The lack of significant electrical distance to the residues from L333C to S341C (Fig. 3A) implies that the MTS reagents do not pass through the electrical field to reach these residues. Login to comment 155 ABCC7 p.Thr351Cys ABCC7 p.Ser341Cys The electrical distance increases dramatically from S341C to T351C (Fig. 3A), suggesting that most of the electrical potential falls in this region of the channel. Login to comment 159 ABCC7 p.Gln353Cys ABCC7 p.Thr351Cys The largest electrical distances that we measured, to T351C and Q353C, was only 0.6. Login to comment