Aim
To characterize phosphorylation of human glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and mobility of GAPDH in cancer cells treated with chemotherapeutic agents.
Conclusion
Our results suggest an important functional role of phosphorylated amino acids in the NAD(+) binding center in GAPDH interactions with its intranuclear partners.
Methods
We used proteomics analysis to detect and characterize phosphorylation sites within human GAPDH. Site-specific mutagenesis and alanine scanning was then performed to evaluate functional significance of phosphorylation sites in the GAPDH polypeptide chain. Enzymatic properties of mutated GAPDH variants were assessed using kinetic studies. Intranuclear dynamics parameters (diffusion coefficient and the immobile fraction) were estimated using fluorescence recovery after photobleaching (FRAP) experiments and confocal microscopy. Molecular modeling experiments were performed to estimate the effects of mutations on NAD(+) cofactor binding.
Results
Using MALDI-TOF analysis, we identified novel phosphorylation sites within the NAD(+) binding center of GAPDH at Y94, S98, and T99. Using polyclonal antibody specific to phospho-T99-containing peptide within GAPDH, we demonstrated accumulation of phospho-T99-GAPDH in the nuclear fractions of A549, HCT116, and SW48 cancer cells after cytotoxic stress. We performed site-mutagenesis, and estimated enzymatic properties, intranuclear distribution, and intranuclear mobility of GAPDH mutated variants. Site-mutagenesis at positions S98 and T99 in the NAD(+) binding center reduced enzymatic activity of GAPDH due to decreased affinity to NAD(+) (Km = 741 ± 257 μmol/L in T99I vs 57 ± 11.1 µmol/L in wild type GAPDH. Molecular modeling experiments revealed the effect of mutations on NAD(+) binding with GAPDH. FRAP (fluorescence recovery after photo bleaching) analysis showed that mutations in NAD(+) binding center of GAPDH abrogated its intranuclear interactions.