The porcine dominant mutation (denoted RN ¯) resulting in a ~ 70 % increase in muscle glycogen content with large effects on meat quality and processing yields was recently found to located in the PRKAG3 gene encoding a muscle specific isoform of the regulatory γ3 subunit of adenosine monophosphate activated protein kinase (AMPK) (Milan et al., 2000). The RN ¯ genotype was found to possess a single nucleotide substitution from CGA to CAA in codon 200 resulting in a nonconservative amino acid substitution from an arginine to a glutamine (Milan et al., 2000).AMPK is a heterotrimer consisting of one catalytic (α) and two regulatory subunits (β and γ). Two isoforms of the catalytic subunit (α1 and α2) have been described while the regulatory subunits are present in three isoforms (β1-3 and γ1-3). The exact physiological relevance of these isoforms has not been completely described, but the γ3 isoform have been found to be musclespecific (Cheung et al., 2000) consistent with RN ¯ animals showing high glycogen content in muscle but not in liver (Estrade et al., 1993). AMPK acts as a metabolic sensor that monitors cellular energy levels and when activated, AMPK inhibits ATP consuming pathways and stimulates ATP regenerating pathways (Hardie and Carling, 1997). AMPK is allosterically activated by an increase in the AMP:ATP and cratine:phosphocreatine ratios which promotes its phosphorylation by an upstream kinase (AMPKK) and inhibits its dephosphorylation (Hardie et al., 1998 ; Kemp et al., 1999). AMPK has been shown to inhibit enzymes involved in glycogen, fatty acid and cholesterol synthesis and to stimulate fatty acid oxidation, glycolysis and glucose uptake (Holmes et al., 1999 ; Kemp et al., 1999 ; Winder and Hardie, 1999 ; Ojuka et al., 2000).
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 2002. Session 11, , 11.2, 2002
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