A novel glycine receptor β subunit splice variant predicts an unorthodox transmembrane topology -: Assembly into heteromeric receptor complexes

The inhibitory glycine receptor is a ligand-gated ion channel with a pentameric assembly from ligand binding alpha and structural beta subunits. In addition to alpha subunit gene variants (alpha 1-alpha 4) and developmental alterations in subunit composition of the receptor protein complex, alternative splicing of alpha subunits has been found to contribute to glycine receptor heterogeneity. Here, we describe a novel splice variant of the glycine receptor beta subunit from mouse central nervous system, prevailing in macroglial cells, predominantly in astrocytes and extraneural tissues. As predicted by its cDNA sequence, the novel subunit beta Delta 7 lacks amino acid positions 251-302 encoded by exon 7 of the Glrb gene. Transcripts and antigen of beta Delta 7 were detected in cerebral cortex, liver, and heart. Lack of exon 7 results in a profoundly altered prediction of transmembrane topology as beta Delta 7 lacks TM1 and TM2 present in the full-length variant. Despite these topological alterations, in vitro studies showed that the beta Delta 7 polypeptide integrates into the plasma membrane, forming receptor complexes with the alpha 1 subunit and gephyrin. Our data demonstrate that a topology deviating from the classical four transmembrane-fold is compatible with formation of glycine receptor protein complexes. However, co-expression of alpha 1 with beta Delta 7 subunits did not change glycine receptor channel properties. Rather, the high level of expression in non- neuronal cells having intimate contact with synaptic regions may account for a yet unknown function of this splice variant beta Delta 7 in glycinergic neurotransmission.