Dissection of the Relationship Between RACK1 and Heterotrimeric G-Proteins in Arabidopsis

Mammalian receptor for activated C kinase 1 (RACK1) is a versatile scaffold protein, playing regulatory roles in multiple signal transduction pathways. Moreover, RACK1 interacts with the heterotrimeric G-proteins (G-proteins) and regulates some specific functions of G beta gamma. Although the protein sequences of both RACK1 and G-proteins are highly conserved in Arabidopsis, their relationship remains elusive. Here we provide genetic and biochemical evidence that Arabidopsis RACK1 and G-proteins may act through a mechanism that is distinct from their counterparts in mammals. Loss-of-function alleles of RACK1A (the most abundantly expressed RACK1 gene in Arabidopsis) do not appear to share morphological and developmental phenotypes with loss-of-function alleles of GPA1 (encoding the sole G alpha in Arabidopsis) or AGB1 (encoding the sole G beta in Arabidopsis). The analysis of gpa1 rack1a and agb1 rack1a double mutants suggested that the effect of RACK1A on morphological and developmental traits may occur independently of the presence or absence of the G-protein subunits. Although both RACK1A and G-protein subunits are negative regulators of ABA responses in the ABA inhibition of early seedling development, an additive ABA hypersensitivity was observed in gpa1 rack1a and agb1 rack1a double mutants. Biochemical analysis suggested that unlike their counterparts in mammals, RACK1 may not physically interact with AGB1. Taken together, these findings revealed some fundamental differences in the relationship of RACK1 and G-proteins between Arabidopsis and mammals.