A parallel three stranded alpha-helical bundle at the nucleation site of collagen triple-helix formation.

A short stretch of 35 amino acids is identified as the structural motif responsible for the tight parallel association and trimerization of the three identical polypeptide chains of lung surfactant protein D, which contains both collagen regions and C-type lectin domains. This 'neck-region' is located at the nucleation site at which the collagenous sequences fold into a staggered triple-helix and is shown, by CD, NMR, and cross-linking of recombinant peptides, to consist of a triple-stranded parallel alpha-helical bundle in a non-staggered, and extremely strong, non-covalent association. This type of association between three polypeptide chains may represent a common structural feature immediately following the C-terminal end of the triple-helical region of collagenous proteins.