Light-sensitive phosphorylation regulates retinal IMPDH1 activity and filament assembly

Area of Science:

• Biochemistry
• Molecular Biology
• Retinal Physiology

Background:

• Inosine monophosphate dehydrogenase (IMPDH) is crucial for guanosine triphosphate (GTP) synthesis, forming filaments that enhance nucleotide production.
• Vertebrate retinas have two IMPDH1 splice variants (IMPDH1(546) and IMPDH1(595)).
• Serine 477 (S477) phosphorylation in bovine retinas is dark-dependent, but its functional impact is unknown.

Purpose of the Study:

• To investigate the structural and functional effects of S477 phosphorylation on IMPDH1 activity and filament assembly.
• To understand the regulatory role of S477 phosphorylation in retinal GTP synthesis.

Main Methods:

• Generation of phosphomimetic S477D mutants for both IMPDH1 variants.
• Cryo-electron microscopy (Cryo-EM) to determine structural changes.
• Cellular assays to assess dominant-negative effects and filament assembly.

Main Results:

• S477D mutation resensitized both IMPDH1 variants to GTP feedback inhibition.
• S477D specifically disrupted the high-activity assembly interface of IMPDH1(595) but allowed IMPDH1(546) filament formation.
• S477D exhibited a dominant-negative effect in cells, inhibiting endogenous IMPDH filament assembly.

Conclusions:

• S477 phosphorylation modulates IMPDH structure and filament assembly, downregulating GTP synthesis in the dark.
• This phosphorylation serves as a regulatory mechanism to decrease nucleotide turnover when cellular demand is low.