Stable changes in expression or activation of G protein αi or αq subunits affect the expression of both β1 and β2 subunits

AbstractG proteins consist of three subunits: α,β and γ. Four β subunits have been cloned: β1 and β4 (36 kDa), and β2 and β3 (35 kDa). We studied endogenous β subunits in mouse NIH 3T3 fibroblasts stably expressing high levels of G protein α subunits after transfection with cDNAs encoding αi1, αi2, αi3 and αq. Immunoblots showed that NIH 3T3 cells express β36 and β35 subunits; in these cells,β35 subunits are four times more abundant than β36 subunits. We could detect β1 and β2 mRNA, but neither β3 norβ4 mRNA. We found that a stable increase in expression of wild-type αi1, αi2, αi3or αq subunits is always accompanied by an increase in β1 and β2 mRNA and protein levels. There was no evidence of selectivity for an increase in β1 rather than β2 subunits depending on the type of α subunit overexpressed. However, constitutive activation or inactivation of α subunits induced specific changes in β subunits. Expression of constitutively inactivated αi2 subunits was accompanied by an increase in mRNA and protein levels of both β subunits. In contrast, cells expressing constitutively activated αi2 subunits did not show any change in the amount of β proteins expressed in membranes, despite a significant increase in β1 and β2 mRNA. We conclude that stable changes in the levels of expression or degree of activation of Gα subunits affect the level of expression, and possibly the turn-over, of β subunits, without selectivity among β1 and β2 subunits