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

The periodical D-band pattern is generally considered a unique ultrastructu ral feature shared by all fibril-forming collagens, which correlates with t he intrafibril, paracrystalline array of tropocollagen monomers. Distinct b and patterns have been reported, however, for collagen stained long-spacing (SLS) crystallites of generic types I, II, and III. Moreover, D-band patte rns 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) t hese distinctive features, (bl tropocollagen heterotrimeric conditions (typ e Ii vs homotrimeric conditions (type II), and (c) different lengths and po or homology between extrahelical telopeptides, the molecular array or telop eptide conformation within the extensively studied type I collagen fibrils could be not the same as those in the very much less intensively studied ty pe II collagen fibrils In this investigation, a distinctive positive staini ng D-band pattern was found fur type If collagen fibrils obtained from huma n cartilages. A fibril model was developed by analyzing actual D-band patte rns, and matching them against simulated patterns based on the primary stru cture of extrahelical and helical domains in human type II tropocollagen. I n particular; a more prominent b(1) band was apparent ill native type (I co llagen fibrils than in. type I. This distinctive feature was also observed for native-type collagen fibrils reconstituted from purified type II collag en, i.e., free from associated minor type XI collagen. On modeling possible monomer arrays, the best fit between microdensitograms and simulation trac es 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 resi dues far band b(1), consistent with the higher electrondensity observed for this band in type II collagen fibrils. N- and C-telopeptide displacement i n 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(2)-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 predic tions of protein secondary structure, according to Choir and Fasman and Ras t and Sander criteria, were consistent with these conformational models, an d suggest that an additional molecular reversal may occur at positions 3N-5 N. These telopeptide "S-fold" conformations, interpreted as axial projectio ns of tridimensional conformation, may represent starting points for furthe r investigation into the still unresolved tridimensional conformation of te lopeptides in monomers arrayed within type II collagen fibrils. (C) 2000 Jo hn Wiley & Sons, Inc.