page_title: Helix bundle
section_title: Three-helix bundles
hierarchical_section_title: Helix bundle / Three-helix bundles
caption_attribution_description: An example of a three-helix bundle protein fold, the headpiece from chicken villin (PDB ID 1QQV).
context_page_description: A helix bundle is a small protein fold composed of several alpha helices that are usually nearly parallel or antiparallel to each other.
context_section_description: Three-helix bundles are among the smallest and fastest known cooperatively folding structural domains. The three-helix bundle in the villin headpiece domain is only 36 amino acids long and is a common subject of study in molecular dynamics simulations because its microsecond-scale folding time is within the timescales accessible to simulation. The 40-residue HIV accessory protein has a very similar fold and has also been the subject of extensive study. There is no general sequence motif associated with three-helix bundles, so they cannot necessarily be predicted from sequence alone. Three-helix bundles often occur in actin-binding proteins and in DNA-binding proteins.
caption: An example of the three-helix bundle fold, the headpiece domain from the protein villin as expressed in chickens (PDB ID 1QQV).
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