The M3/M4 cytoplasmic loop of the α1 subunit restricts GABAARs lateral mobility:: A study using fluorescence recovery after photobleaching

A crucial problem in neurobiology is how neurons are able to maintain neurotransmitter receptors at specific membrane domains. The large structural heterogeneity of gamma aminobutyric acid receptors (GABA(A)Rs) led to the hypothesis that there could be a link between GABA(A)R gene diversity and the targeting properties of the receptor complex. Previous studies using Fluorescence Recovery After Photobleaching (FRAP) have shown a restricted mobility in GABAARs containing the alpha 1 subunit. The M3/M4 cytoplasmic loop is the region of the alpha 1 subunit with the lowest sequence homology to other subunits. Therefore, we asked whether the M3/M4 loop is involved in cytoskeletal anchoring and GABAAR clustering. A series of alpha 1 chimeric subunits was constructed: alpha 1CH (control subunit), alpha 1CD (Cytoplasmic loop deleted), alpha 1CD2, and alpha 1CD3 (alpha 1 with the M3/M4 loop from the alpha 2 and alpha 3 subunits, respectively). Our results using FRAP indicate an involvement of the M3/M4 cytoplasmic loop of the alpha 1 subunit in controlling receptor lateral mobility. On the other hand, immmocytochemical approaches showed that this domain is not involved in subunit targeting to the cell surface, subunit-subunit assembly, or receptor aggregation.