SEQUENCE OF A FUNCTIONAL INVERTEBRATE GABA-A RECEPTOR SUBUNIT WHICH CAN FORM A CHIMERIC RECEPTOR WITH A VERTEBRATE ALPHA-SUBUNIT

The sequence of an invertebrate GABA(A) receptor subunit is described. This was deduced from a cDNA which was isolated from the mollusc Lymnaea stagnalis and which corresponds to a transcript of extremely low abundance. The cDNA was isolated using short exonic sequences from part of the corresponding gene in combination with a variant of the polymerase chain reaction (PCR) known as RACE (rapid amplification of cDNA ends). The mature polypeptide has a predicted molecular weight of 54 569 Daltons and exhibits approximately 50% identity to vertebrate GABA(A) receptor beta-subunits. The six intron-exon boundaries determined to date in the molluscan gene occur at the same relative positions as those found in vertebrate GABA(A) receptor genes. Functional expression, in Xenopus oocytes, of the molluscan cDNA alone results in the formation of GABA-activated chloride ion channels that have a finite open probability even in the absence of agonist. These GABA-evoked currents can be reversibly blocked by the vertebrate GABA(A) receptor antagonist bicuculline. Surprisingly, the molluscan beta-subunit is capable of replacing vertebrate beta-subunits in co-expression experiments with the bovine GABA(A) receptor alpha-1 subunit. These findings suggest that invertebrate GABA(A) receptors exist in vivo as hetero-oligomeric complexes.