Question

Patients with the syndrome pseudohypoparathyroidism have an inactivating mutation (R228C) of one allele of the Gαs isoform of the heterotrimeric G protein. On the other hand, gain-of-function point mutation R201C is the most common cancer-causing mutation found in heterotrimeric G proteins. Interestingly, both mutations alter the role of GTP in G protein function. How could these two genetic changes result in such dramatic changes in protein function?

Answer 1

The loss of function mutation R228C in G-alpha subunits leads to pseudohypoparathyroidism type 1a (PHP-Ia). Loss-of-function mutations in the H-bond network of G-alpha subunits that cause pseudohypoparathyroidism are thought to be due to defects in GTP binding as well as hyper GTPase activity. However, recent studies ( Hu et al., 2018) point to the compromised adenylyl cyclase-activating activity of G-alpha subunits bound to a non-hydrolyzable GTP analog as the reason for the phenotype.

The gain of function point mutation R201 leads to the loss of GTPase activity and hence results in the inability to switch off to the GDP state of the G-alpha heteromeric subunit. Also, the R201C mutation can bypass the need for GTP binding by directly activating GDP-bound G-alpha through stabilization of an intramolecular hydrogen bond network thus converting GDP into an activator. The R201 residue is at the site of an intramolecular arginine finger in the nucleotide pocket, and in the GDP-bound state, this residue could control the GDP-bound conformation, which is disrupted by the oncogenic R201C mutation. Recent structural and functional analysis of the R201C G-alpha mutation has discovered the ability of the protein to activate its downstream effector adenylyl cyclase while binding to GDP even in the presence of G-beta-gamma subunits. Thus the R201 residue helps in maintaining GDP-bound Gas in an off state through destabilization of an intramolecular hydrogen bond network (H-bond network) which is lost by the R201C gain-of-function mutation, thus leading to cancer.

Thus disease-causing mutations in G-alpha subunits can subvert the canonical roles of GDP and GTP thus leading to dramatic and different phenotypes.