A model for type II collagen fibrils: Distinctive D‐band patterns in native and reconstituted fibrils compared with sequence data for helix and telopeptide domains

Abstract

The periodical D-band pattern is generally considered a unique ultrastructural feature shared by all fibril-forming collagens, which correlates with the intrafibril, paracrystalline array of tropocollagen monomers. Distinct band patterns have been reported, however, for collagen stained long-spacing (SLS) crystallites of genetic types I, II, and III. Moreover, D-band patterns of negatively stained, native type II collagen fibrils were found to be not identical to those of type I in our previous research. Because of (a) these distinctive features, (b) tropocollagen heterotrimeric conditions (type I) vs homotrimeric conditions (type II), and (c) different lengths and poor homology between extrahelical telopeptides, the molecular array or telopeptide conformation within the extensively studied type I collagen fibrils could be not the same as those in the very much less intensively studied type II collagen fibrils. In this investigation, a distinctive positive-staining D-band pattern was found for type II collagen fibrils obtained from human cartilages. A fibril model was developed by analyzing actual D-band patterns, and matching them against simulated patterns based on the primary structure of extrahelical and helical domains in human type II tropocollagen. In particular, a more prominent b(1) band was apparent in native type II collagen fibrils than in type I. This distinctive feature was also observed for native-type collagen fibrils reconstituted from purified type II collagen, i.e., free from associated minor type XI collagen. On modeling possible monomer arrays, the best fit between microdensitograms and simulation traces was found for 234 amino acid staggering, as is also the case for type I collagen fibrils. On comparing this model with an analogous one for type I collagen fibrils, there was a higher intraband distribution of charged residues for band b(1), consistent with the higher electrondensity observed for this band in type II collagen fibrils. N- and C-telopeptide displacement in the model corresponded to D-locations of a c(2) subband, which we named c(2.0), and band a(3), respectively. In simulation profiles, c(2.0) -like and a(3) -like peaks mimicked the corresponding peaks in microdensitograms when molecular reversals were adopted at positions 10N-12N, 12C-14C, and 17C-19C for N- and C-telopeptides. Hydrophobic interactions and algorithmic predictions of protein secondary structure, according to Chou and Fasman and Rost and Sander criteria, were consistent with these conformational models, and suggest that an additional molecular reversal may occur at positions 3N-5N. These telopeptide "S-fold" conformations, interpreted as axial projections of tridimensional conformation, may represent starting points for further investigation into the still unresolved tridimensional conformation of telopeptides in monomers arrayed within type II collagen fibrils.

In Situ D-periodic Molecular Structure of Type II Collagen

“…In contrast, the proposed collagen type II C-telopeptide structure is much closer to that observed for the collagen type I C-telopeptide and to the prediction of Ortolani et al (39). It has tyrosine (Tyr-1058) and methionine (Met-1055) residues at the end that have been moved deeper into the overlap region, closer to the N terminus.…”

“…Electron microscopy studies of human collagen type II fibers suggested that the folding of the telopeptides is similar to that proposed here (39), but the 20 nm resolution of those studies was not sufficient to provide conclusive evidence (the stretched N-and C-telopeptides are 4.5 nm and 5.4 nm in length, respectively). Similarly, studies with synthetic or ex vivo collagen N-and C-telopeptides indicated an axial compression of the fragment(s), with possible folding (52-54).…”

“…Previous investigations of the structure of fibrillar collagen indicated that the telopeptides form contracted and folded structures (9,17,21,39). Electron microscopy studies of human collagen type II fibers suggested that the folding of the telopeptides is similar to that proposed here (39), but the 20 nm resolution of those studies was not sufficient to provide conclusive evidence (the stretched N-and C-telopeptides are 4.5 nm and 5.4 nm in length, respectively).…”

“…However, the overlap is 0.42D Ϯ 0.03 (28.14 nm or 98-amino acid residues long), and the gap is 0.58D (38.86 nm or 136 amino acid residues) in type II collagen, whereas in type I collagen, the overlap is 0.46D and gap is 0.54D. The overlap/gap ratio for type II collagen has previously been suggested to be closer to 0.4D/0.6D than type I (39,40,44,45) and is also confirmed from the TEM data ( Fig. 5 and supplemental Fig.…”

“…4). The collagen fibers in these images show the typical pattern for fixed collagenous tissues: black and white bands with ϳ64 nm periodicity (D-period shortened during fixation and embedding (15,39)). These images also confirm previous observations that collagen type II fibrils from lamprey notochord do not show any detectable differences from those of mammalian tissues (15,16).…”