The Gpn3 Q279* cancer-associated mutant inhibits Gpn1 nuclear export and is deficient in RNA polymerase II nuclear targeting.

Gpn3 is required for RNA polymerase II (RNAPII) nuclear targeting. Here, we investigated the effect of a cancer-associated Q279* nonsense mutation in Gpn3 cellular function. Employing RNAi, we replaced endogenous Gpn3 by wt or Q279* RNAi-resistant Gpn3R in epithelial model cells. RNAPII nuclear accumulation and transcriptional activity were markedly decreased in cells expressing only Gpn3R Q279*. Wild-type Gpn3R localized to the cytoplasm but a fraction of Gpn3R Q279* entered the cell nucleus and inhibited Gpn1-EYFP nuclear export. This property and the transcriptional deficit in Gpn3R Q279*-expressing cells required a PDZ-binding motif generated by the Q279* mutation. We conclude that an acquired PDZ-binding motif in Gpn3 Q279* caused Gpn3 nuclear entry, and inhibited Gpn1 nuclear export and Gpn3-mediated RNAPII nuclear targeting.

Human Gpn1 purified from bacteria binds guanine nucleotides and hydrolyzes GTP as a protein dimer stabilized by its C-terminal tail.

The essential GTPase Gpn1 mediates RNA polymerase II nuclear targeting and controls microtubule dynamics in yeast and human cells by molecular mechanisms still under investigation. Here, we purified human HisGpn1 expressed as a recombinant protein in bacteria E. coli BL-21 (DE3). Affinity purified HisGpn1 eluted from a size exclusion column as a protein dimer, a state conserved after removing the hexa-histidine tail and confirmed by separating HisGpn1 in native gels, and in dynamic light scattering experiments. Human HisGpn1 purity was higher than 95%, molecularly monodisperse and could be concentrated to more than 10 mg/mL without aggregating. Circular dichroism spectra showed that human HisGpn1 was properly folded and displayed a secondary structure rich in alpha helices. HisGpn1 effectively bound GDP and the non-hydrolyzable GTP analogue GMPPCP, and hydrolyzed GTP. We next tested the importance of the C-terminal tail, present in eukaryotic Gpn1 but not in the ancestral archaeal Gpn protein, on HisGpn1 dimer formation. C-terminal deleted human HisGpn1 (HisGpn1ΔC) was also purified as a protein dimer, indicating that the N-terminal GTPase domain contains the interaction surface needed for dimer formation. In contrast to HisGpn1, however, HisGpn1ΔC dimer spontaneously dissociated into monomers. In conclusion, we have developed a method to purify properly folded and functionally active human HisGpn1 from bacteria, and showed that the C-terminal tail, universally conserved in all eukaryotic Gpn1 orthologues, stabilizes the GTPase domain-mediated Gpn1 protein dimer. The availability of recombinant human Gpn1 will open new research avenues to unveil the molecular and pharmacological properties of this essential GTPase.