The N-terminal domain of human GABA receptor ρ1 subunits contains signals for homooligomeric and heterooligomeric interaction

γ-Aminobutyric acid type C (GABA(C)) receptors identified in retina appear to be composed of GABA ρ sub-units. The purpose of this study was to localize signals for homooligomeric assembly of ρ1 subunits and to investigate whether the same region contained signals for heterooligomeric interaction with ρ1 subunits. In vitro translated human ρ1 was shown to be membrane-associated, and proteinase K susceptibility studies indicated that the N terminus was oriented in the lumen of ER-derived microsomal vesicles. This orientation suggested the involvement of the N terminus of ρ1 in the initial steps of subunit assembly. To test this hypothesis, mutants were created containing only N-terminal sequences (N-ρ1) or C-terminal sequences (C-ρ1) of ρ1. Coimmunoprecipitation studies revealed that N-ρ1, but not C- ρ1, interacted with ρ1 in vitro. When coexpressed in Xenopus oocytes, N- ρ1 interfered with ρ1 receptor formation. Together, these data suggested that signals for ρ1 homooligomeric assembly reside in the N-terminal half of the subunit. Sequential immunoprecipitations were then performed upon cotranslated ρ1 and ρ2 sub-units which demonstrated that ρ1 and ρ2 interacted in vitro. Co-immunoprecipitation indicated that N-ρ1 specifically associated with ρ2. Therefore, the N-terminal regions of p subunits contain the initial signals for both homooligomeric and heterooligomeric assembly into receptors with GABA(C) properties.