The N-terminal domain of human GABA receptor rho 1 subunits contains signals for homooligomeric and heterooligomeric interactions

gamma-aminobutyric acid type C (GABA(C)) receptors identified in retina appear to be composed of GABA rho subunits. The purpose of this study was to localize signals for homooligomeric assembly of rho 1 subunits and to investigate whether the same region contained signals for heterooligomeric interaction with rho 2 subunits. In vitro translated human rho 1 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 rho 1 in the initial steps of subunit assembly. To test this hypothesis, mutants were created containing only N-terminal sequences (N-rho 1) or C-terminal sequences (C-rho 1) of rho 1. Co-immunoprecipitation studies revealed that N-rho 1, but not C-rho 1, interacted with rho 1 in vitro. When coexpressed in Xenopus oocytes, N-rho 1 interfered with rho 1 receptor formation. Together, these data suggested that signals for rho 1 homooligomeric assembly reside in the N-terminal half of the subunit. Sequential immunoprecipitations were then performed upon cotranslated rho 1 and rho 2 subunits which demonstrated that rho 1 and rho 2 interacted in vitro. Co-immunoprecipitation indicated that N-rho 1 specifically associated with rho 2. Therefore, the N-terminal regions of rho subunits contain the initial signals for both homooligomeric and heterooligomeric assembly into receptors with GABA(C) properties.