The GNMT N-terminus Couples Folate Feedback to Methyl-donor Homeostasis.

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• S-adenosylmethionine (SAM) homeostasis is vital for cellular functions, including methylation, nucleotide, and polyamine synthesis.
• Glycine N-methyltransferase (GNMT), primarily in the liver, plays a critical role in SAM homeostasis and is regulated by 5-methyltetrahydrofolate (5mTHF).

Purpose of the Study:

• To investigate the regulatory role of the GNMT N-terminal tail and its phosphorylation at serine 9 (S9ph) in SAM homeostasis and folate-dependent feedback inhibition.
• To elucidate the structural and functional consequences of N-terminal modifications on GNMT activity and its interaction with 5mTHF.

Main Methods:

• Structural and biochemical analyses
• Molecular dynamics simulations
• Phosphoproteomic analysis in mouse liver
• Lentiviral overexpression in hepatocyte cell lines

Main Results:

• The GNMT N-terminal tail is essential for SAM turnover and 5mTHF binding.
• GNMT S9ph is abundant in mouse liver, increasing with age, and abolishes 5mTHF binding when mimicked or truncated.
• Overexpression of active GNMT mutants depleted SAM, increased S-adenosylhomocysteine (SAH), disrupted protein methylation, and induced methyl-donor stress responses.

Conclusions:

• The GNMT N-terminus acts as a phosphoregulatory domain, dynamically controlling GNMT activity.
• Phosphorylation of GNMT at S9 links folate-dependent feedback to SAM homeostasis, impacting cellular methylation potential.
• Dysregulated GNMT activity due to altered N-terminal regulation may contribute to compromised liver function.