A new twist in the collagen story--the type VI segmented supercoil

Collagen occurs in two major forms: fibrillar and non-fibrillar. Non-fibrillar collagens are structurally more variable and relatively ill-understood. In this work we analysed the amino acid sequence of type VI collagen, a non-fibrillar collagen that forms antiparallel dimers. A sequence motif was discovered that gives rise to systematic molecular coiling. There is a common periodicity ( approximately 23 or 2 x 23 residues) in the charged amino acids, in the prolines and in the discontinuities in the Gly-X-Y triplets. In addition, there is a different periodicity ( approximately 21 amino acids) in the apolar groups. The two repeats mean that the only way to simultaneously maximize both the hydrophobic and polar interactions during dimer formation is with the molecules antiparallel, overlapped by 75 nm as observed, and supercoiled. The alternating proline-rich and charge-rich patches, often together with discontinuities in the Gly-X-Y sequences, coincide with each half-turn of the supercoil, thus breaking it into segments. We have termed this structure the collagen segmented supercoil. The segmented supercoil and variants may be common aggregation motifs for the non-fibrillar collagens.