AMP/ATP-binding subunit of AMP-activated protein kinase (AMPK), an energy sensor protein kinase that plays a key role in regulating cellular energy metabolism. In response to reduction of intracellular ATP levels, AMPK activates energy-producing pathways and inhibits energy-consuming processes: inhibits protein, carbohydrate and lipid biosynthesis, as well as cell growth and proliferation. AMPK acts via direct phosphorylation of metabolic enzymes, and by longer-term effects via phosphorylation of transcription regulators. AMPK also acts as a regulator of cellular polarity by remodeling the actin cytoskeleton; probably by indirectly activating myosin. The AMPK gamma3 subunit is a non-catalytic subunit with a regulatory role in muscle energy metabolism (PubMed:17878938). It mediates binding to AMP, ADP and ATP, leading to AMPK activation or inhibition: AMP-binding results in allosteric activation of alpha catalytic subunit (PRKAA1 or PRKAA2) both by inducing phosphorylation and preventing dephosphorylation of catalytic subunits. ADP also stimulates phosphorylation, without stimulating already phosphorylated catalytic subunit. ATP promotes dephosphorylation of catalytic subunit, rendering the AMPK enzyme inactive. AMPK is a heterotrimer of an alpha catalytic subunit (PRKAA1 or PRKAA2), a beta (PRKAB1 or PRKAB2) and a gamma non-catalytic subunits (PRKAG1, PRKAG2 or PRKAG3). Interacts with FNIP1 and FNIP2 (By similarity). Skeletal muscle, with weak expression in heart and pancreas. The AMPK pseudosubstrate motif resembles the sequence around sites phosphorylated on target proteins of AMPK, except the presence of a non-phosphorylatable residue in place of Ser. In the absence of AMP this pseudosubstrate sequence may bind to the active site groove on the alpha subunit (PRKAA1 or PRKAA2), preventing phosphorylation by the upstream activating kinase STK11/LKB1. The 4 CBS domains mediate binding to nucleotides. Of the 4 potential nucleotide-binding sites, 3 are occupied, designated as sites 1, 3, and 4 based on the CBS modules that provide the acidic residue for coordination with the 2'- and 3'-hydroxyl groups of the ribose of AMP. Of these, site 4 appears to be a structural site that retains a tightly held AMP molecule (AMP 3). The 2 remaining sites, 1 and 3, can bind either AMP, ADP or ATP. Site 1 (AMP, ADP or ATP 1) is the high-affinity binding site and likely accommodates AMP or ADP. Site 3 (AMP, ADP or ATP 2) is the weakest nucleotide-binding site on the gamma subunit, yet it is exquisitely sensitive to changes in nucleotide levels and this allows AMPK to respond rapidly to changes in cellular energy status. Site 3 is likely to be responsible for protection of a conserved threonine in the activation loop of the alpha catalytic subunit through conformational changes induced by binding of AMP or ADP. Phosphorylated by ULK1; leading to negatively regulate AMPK activity and suggesting the existence of a regulatory feedback loop between ULK1 and AMPK. Genetic variation in PRKAG3 defines the skeletal muscle glycogen content and metabolism quantitative trait locus (SMGMQTL) [MIM:619030]. Muscle fibers from carriers of variant Trp-225 have approximately 90% more muscle glycogen content than controls and decreased levels of intramuscular triglyceride. Belongs to the 5'-AMP-activated protein kinase gamma subunit family.