A PARALLEL 3 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 moti f responsible for the tight parallel association and trimerization of the three identical polypeptide chains of lung surfactant protein D, w hich contains both collagen regions and C-type lectin domains. This 'n eck-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-st randed parallel cc-helical bundle in a non-staggered, and extremely st rong, non-covalent association. This type of association between three polypeptide chains may represent a common structural feature immediat ely following the C-terminal end of the triple-helical region of colla genous proteins.