Methyl groups are thought to dominate the dynamics of proteins after slow collective modes of motion freeze out in a glass-transition process. In this work we investigate methyl group dynamics of a key hydrophobic core leucine residue in chicken villin headpiece subdomain protein at 140-4 K using deuteron NMR longitudinal relaxation measurements. A distinct increase in the apparent activation energy is observed at similar to 95 K, indicating an abrupt freezing of methyl group dynamics. Relaxation times at temperatures below 60 K are dominated by the deuteron tunneling mechanism.