Ric-8 folding of G proteins better explains Ric-8 apparent amplification of G protein–coupled receptor signaling

In their PNAS article, Kataria et al. conclude that “…Dictyostelium resistant to inhibitors of cholinesterase 8 (Ric-8), in contrast to mammalian Ric-8, does not play a role in the expression or folding of G proteins.” (1) We contend that the experimental evidence presented to support this claim is not sufficient. Figure 5A reported similar steady-state whole cell Gα2 levels in a ric-8-null organism vs. a WT control. This experiment reveals no information about the functionality or folded state of Gα2 or any other Gα subunit in ric-8-null Dictyostelia. In Ric-8A-null mouse embryonic stem (ES) cell whole cell lysates, we reported slight reductions of particular Gα subunits (e.g., Gαo and Gα13) and dramatic reductions of others (e.g., Gαq and Gαi_1/2) (2). However, Gαo and Gα13 were reduced dramatically in a subcellular membrane fraction prepared from Ric-8A-null cells because these G proteins were not folded properly during biosynthesis (2, 3). The individual stabilities of misfolded Gα subunits differ depending on subtype. Gα subunit misfolded status in Ric-8 absence may not always be revealed by immunoblotting Gα subunits from whole cell lysates.

More than 10 published reports have provided evidence that Ric-8 homologs in human cells, mice, Drosophila, Caenorhabditis elegans, Neurospora, and Spodoptera frugiperda influence the functional abundance of endogenous and recombinant G protein α subunits (references withheld because of limit). If Dictyostelium Ric-8 is to be considered unique among all of these investigated Ric-8 homologs and lack any ability to participate in Dictyostelium G protein biosynthetic folding, more evidence must be provided to support this claim.

The authors make the summary conclusion that the “relevant” function of mammalian and Dictyostelium Ric-8 proteins in G protein signaling is to act as (re)amplifiers of G proteins that have been activated by G protein–coupled receptors (GPCRs). All of the results in which Kataria et al. observed clear G protein signaling aberrancies in ric-8-null Dictyostelia might be explained if Ric-8 was acting as a signal amplifier, but these observations are explained equally as well (if not better) if the expressed G proteins in ric-8-null Dictyostelia had dysfunctional folds. More evidence is required to discriminate the chaperoning and putative signal amplification functions of Ric-8 proteins in all species. It is imprudent to discount the relevancy of either of these activities at this time.