A first census of collagen interruptions: collagen's own stutters and stammers.

The repetitive Gly-X-Y sequence is the telltale sign of triple helical domains in collagens and collagen-like proteins. Most collagen sequences contain sporadic interruptions of this pattern, which may have functional roles in molecular flexibility, assembly or molecular recognition. However, the structural signatures of the different interruptions are not well defined. Here, a first comprehensive survey of collagen interruptions on collagen sequences from different taxonomic groups is presented. Amino acid preferences at the sites of interruption and the flanking triplets are analysed separately for metazoan and prokaryotic collagens and the concept of commensurateness between interruptions is introduced. Known structural information from model peptides is used to present a common framework for hydrogen bonding topology and variations in superhelical twist for the different types of interruptions. Several collagen interruptions are further classified here as stutters or stammers in analogy to the heptad breaks observed in alpha-helical coiled coils, and the structural consequences of commensurate interruptions in heterotrimeric collagens are briefly discussed. Data presented here will be useful for further investigation on the relation between structure and function of collagen interruptions.