Mechanisms of collagen trimer assembly

It is generally accepted that the folding of collagen triple helical domains occur from the C-terminus toward the N-terminus by a “zipper” mechanism. The regions at the C-terminus of the triple helices must therefore play a critical role in the processes of chain recognition and assembly to get the proper stoichiometries and of chain registration to align the chains for the folding of the triple helix. Examination of these regions reveals a broad diversity of structures and suggests that different mechanisms of assembly are used in the various collagen and collagen-like molecules. We review here three different mechanisms that have recently come to light. The collectins, a group of serum proteins containing collagen-like triple helical domains, are assembled through hydrophobic interactions in a tripleα helix. Collagens VIII and X, C1q and several related proteins contain homologous C-terminal domains that are characterized by aβ-pleated sheet structure. They assemble through very strong hydrophobic interactions that probably involve an “aromatic zipper”. Collagens IX, XII and XIV fibril associated collagen with interrupted triple helices (FACITs), are assembled by a mechanism in which both the C-terminal triple helix and a very short cysteine-containing sequence are involved.