The tunnel for GSA channeling. The yellow dashed line indicates the proposed pathway of GSA. Gly101, Cys144, Arg146, and His193 (corresponding to Gly8, Cys48Ser, Arg50, and His84 in MkGluTR) are shown as sticks and labeled. The glutamate moiety of glutamycin in the MkGluTR structure is shown in yellow stick representation.