The luteinizing hormone receptor activates phospholipase C via preferential coupling to G(i2)

Binding of lutropin/choriogonadotropin (LH/CG) to its cognate receptor resu lts in the activation of adenylyl cyclase and phopholipase C. This divergen t signaling of the LH receptor is based on the independent activation of di stinct G protein subfamilies, i.e., G(s), G(i), and potentially also G(q). To examine the selectivity of LH receptor coupling to phospholipase C beta- activating G proteins, we used an in vivo reconstitution system based on th e coexpression of the LH receptor and different G proteins in baculovirus-i nfected insect cells. In this paper, we describe a refined expression strat egy for the LH receptor in insect cells. The receptor protein was inserted into the cell membrane at an expression level of 0.8 pmol/mg of membrane pr otein. Sf9 cells expressing the LH receptor responded to hCG challenge with a concentration-dependent accumulation of intracellular cAMP (EC50 = 630 n M) but not of inositol phosphates, whereas stimulation of the histamine H-1 receptor in Sf9 cells led to increased phospholipase C (PLC) activity. Imm unoblotting experiments using G protein-specific antisera revealed the abse nce of quantitative amounts of alpha(i) in Sf9 cells, whereas alpha(s) and alpha(q/11) were detected. We therefore attempted to restore the hCG-depend ent PLC activation by infection of Sf9 cells with viruses encoding the LH r eceptor and different G protein a subunits. HCG stimulation of cells coexpr essing the LH receptor and exogenous alpha(i2) resulted in stimulation of P LC activity. In cells coinfected with an alpha(i3)-baculovirus, hCG challen ge led to a minor activation of PLC, whereas no hCG-dependent PLC stimulati on was observed in cells coexpressing alpha(i1). Most notably, coinfection with baculoviruses encoding alpha(q) or alpha(11) did not reproduce the PLC activation by the LH receptor. Thus, the murine LH receptor activates aden ylyl cyclase via G(s) and PLC via selective coupling to G(i2).