Knobs into holes packing is a protein packing motif

Structural motif

In a chain-like biological molecule, such as a protein or nucleic acid, a structural motif is a supersecondary structure, which appears also in a variety of other molecules...

 that occurs mainly in alpha helix

Alpha helix

A common motif in the secondary structure of proteins, the alpha helix is a right-handed coiled or spiral conformation, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier...

 or coiled coil

Coiled coil

A coiled coil is a structural motif in proteins, in which 2-7 alpha-helices are coiled together like the strands of a rope . Many coiled coil type proteins are involved in important biological functions such as the regulation of gene expression e.g. transcription factors...

 domains

Protein domain

A protein domain is a part of protein sequence and structure that can evolve, function, and exist independently of the rest of the protein chain. Each domain forms a compact three-dimensional structure and often can be independently stable and folded. Many proteins consist of several structural...

. One such example is fibrinogen fibril formation.

This motif occurs in coiled coil motifs to maintain and reach a 20 degree "left hand crossing angle that is characteristic of coiled coils". This motif looks much like two springs slammed together. (See also Helix bundle

Helix bundle

A helix bundle is a small protein fold composed of several alpha helices that are usually nearly parallel or antiparallel to each other.-Three-helix bundles:Three-helix bundles are among the smallest and fastest known cooperatively folding structural domains...

)

Further reading

1. Cryst
2. Blood Journal
3. University of Utrecht

Source

• Embase Eilers M., Patel A.B., Liu W. and Smith S.O. (2002). "Comparison of helix interactions in membrane and soluble alpha-bundle proteins." Biophysical Journal. 82(5) (pp 2720–2736).