2 THREONINE RESIDUES IN THE M2 SEGMENT OF THE ALPHA(1)BETA(1) GABA(A)RECEPTOR ARE CRITICAL FOR ION-CHANNEL FUNCTION

The role of three threonine residues in the M2 hydrophobic region of t he GABA(A) receptor has been investigated by replacing these polar res idues with alanine at the 6', 10' or 13' positions of M2 in the GABA(A ) alpha(1) and beta(1) subunits and co-expressing the mutated subunits in the baculovirus Sf9 insect cell system. GABA did not elicit a curr ent in cells expressing either the 6' or 13' threonine to alanine muta nts. The mutant subunits formed intact heteromeric GABA(A) receptors a s judged by the binding of [H-3] muscimol or the relative level of alp ha(1) protein present in the plasma membrane. In contrast, a chloride current was generated by GABA in cells expressing the 10' mutant recep tor. However, the current decayed more rapidly to baseline in the cont inued presence of GABA in the 10' mutant receptor than in the wild typ e receptor. The results are discussed in terms of the possible roles o f the threonine residues in the ion conduction pathway.