Cysteine-scanning analysis of helices TM8, TM9a, and TM9b and intervening loops in the YgfO xanthine permease: a carboxyl group is essential at ASP-276.

Cysteine-scanning analysis of helices TM8, TM9a, and TM9b and intervening loops in the YgfO xanthine permease: a carboxyl group is essential at ASP-276.