Molecular bending and networks in a basement membrane-like collagen: packing in dogfish egg capsule collagen

Molecular and ultrastructural studies of a fibrillar collagen from octocoral (Cnidaria)

We report here the biochemical, molecular and ultrastructural features of a unique organization of fibrillar collagen extracted from the octocoral Sarcophyton ehrenbergi Collagen, the most abundant protein in the animal kingdom, is often defined as a structural component of extracellular matrices in metazoans. In the present study, collagen fibers were extracted from the mesenteries of S. ehrenbergi polyps. These fibers are organized as filaments and further compacted as coiled fibers. The fibers are uniquely long, reaching an unprecedented length of tens of centimeters. The diameter of these fibers is 9±0.37 μm. The amino acid content of these fibers was identified using chromatography and revealed close similarity in content to mammalian type I and II collagens. The ultrastructural organization of the fibers was characterized by means of high-resolution microscopy and X-ray diffraction. The fibers are composed of fibrils and fibril bundles in the range of 15 to 35 nm. These data indicate a fibrillar collagen possessing structural aspects of both types I and II collagen, a highly interesting and newly described form of fibrillar collagen organization.

Structural proteins from whelk egg capsule with long range elasticity associated with a solid-state phase transition

The robust, proteinaceous egg capsules of marine prosobranch gastropods (genus Busycotypus ) exhibit unique biomechanical properties such as high elastic strain recovery and elastic energy dissipation capability. Capsule material possesses long-range extensibility that is fully recoverable and is the result of a secondary structure phase transition from α-helical coiled-coil to extended β-sheet rather than of entropic (rubber) elasticity. We report here the characterization of the precursor proteins that make up this material. Three different proteins have been purified and analyzed, and complete protein sequences deduced from messenger ribonucleic acid (mRNA) transcripts. Circular dichroism (CD) and Fourier transform infrared (FTIR) spectroscopy indicate that the proteins are strongly α-helical in solution and primary sequence analysis suggests that these proteins have a propensity to form coiled-coils. This is in agreement with previous wide-angle X-ray scattering (WAXS) and solid-state Raman spectroscopic analysis of mature egg capsules.

Dogfish egg case structural studies by ATR FT-IR and FT-Raman spectroscopy

The dogfish egg case is a composite structure that combines mechanical tensile strength, toughness and elasticity with high permeability to small molecules and ions. Presumably, it provides both a protective and a filtering role for the egg/embryo contained within it. In this work, we performed structural studies of the Galeus melastomus egg case at two different stages of the hardening process, utilizing ATR FT-IR and FT-Raman spectroscopy. Based on these data we deduce that: (a) The G. melastomus egg case, in close analogy to that of the related species Scyliorhinus cunicula, is a complex, composite structure which consists mainly of an analogue of collagen IV. This network forming protein appears to have common secondary structural characteristics in the entire egg case. (b) The outermost layer of the non-sclerotized egg case is especially rich in tyrosine, while the innermost layer is rich in polysaccharides, presumably glycosaminoglycans, and lipids. These differences are diminished upon hardening. (c) Disulfide bonds do not appear to play a significant role in cross-linking. However, cross-links involving tyrosine residues appear to sclerotize the egg case. It is proposed that the intensity of the Raman band at ca. 1615 cm(-1), which is due to ring stretching vibrations of Tyr, might be a useful indicator of the sclerotization status of a certain proteinaceous tissue, when tyrosines are involved in sclerotization mechanisms.

Characterization of selachian egg case collagen

The egg case of the dogfish Scyliorhinus canicula is a remarkable collagenous structure that combines mechanical strength and toughness with high permeability to small molecules and ions. The collagenous lamellae that form over 80% of the thickness of the case wall are secreted by the D-zone of the nidamental (oviducal gland). An acid-soluble collagen extracted from this zone and partially purified ran as a single band on a native gel at pH 4.3. A single band of identical mobility was extracted from egg cases removed from the oviducal gland. SDS-PAGE of both extracts revealed a major component with an apparent molecular weight of 35 kDa and a minor component at 34 kDa. Neither of these components appeared to be glycosylated. Amino acid analysis of the partially purified collagen extracted from the oviducal gland revealed a composition similar to that of the collagenous lamellae of the egg case with glycine accounting for 16% and imino acids for 10% of the total residues. Partial N-terminal and internal sequences were obtained by Edman degradation for peptides extracted from the D-zone of the nidamental gland. Four of the internal sequence fragments showed repeated G-X-Y triplets showing them to be collagenous. These four fragments were novel but showed similarity to the triple-helical domains of mammalian type IV, X, and VI collagens. The noncollagenous N-terminal and pepsin-resistant sequences were unique, showing no significant similarity to known proteins in the database. Several possible N-myristoylation and phosphorylation sites were identified in the noncollagenous sequences.

Collagen packing in the dogfish egg case wall

The collagen which forms the egg case of the dogfish, Scyliorhinus canicula, is assembled in a three-dimensional network that results in a very resistant capsule. The capsule presumably accomplishes both a protective and a filtering role for the embryo contained within it. In the present work we have obtained electron micrographs of metal-shadowed, deep-etched replicas of the egg case and we have analyzed the micrographs using computer-based Fourier methods. The replicas provide details of the three-dimensional structure that have not been recorded before, allowing us to add particulars to the [1,0,0], [1,1,0], and [0,0,1] views previously obtained from plastic sections (C. Knupp et al., 1996, J. Struct. Biol. 117, 209-221). A molecular packing arrangement consistent with all of the present data is proposed. Although this collagen type is unique and is substantially shorter (45 nm) than other collagens, investigations into its structure may give an insight into related collagen types. The role of this collagen as both a protective structure and one with filtering properties controlling permeability is discussed in terms of other collagens with similar functions.

Molecular orientations in an extruded collagenous composite, the marginal rib of the egg capsule of the dogfish Scyliorhinus canicula ; a novel lyotropic liquid crystalline arrangement and its origin in the spinnerets

The egg case of the dogfish, Scyliorhinus canicula is a composite material largely constructed from collagen fibrils. It is formed as a drawn extrusion from transverse rows of spinnerets within the lining of the nidamental gland. In the L 2 layer, which forms over 90% of the thickness of the marginal rib, each spinneret extrudes a flattened ribbon which runs the length of the rib and measures approximately 150 x 8.5 μm in cross section. The structure of these ribbons and the orientation of collagen molecules and fibrils within them has been investigated in a correlative study using: low angle x-ray diffraction; bright field microscopy of peeled preparations; transmission (TEM) and scanning (SEM) electron microscopy; confocal and quantitative polarizing microscopy. The way in which the molecular orientations are defined within the spinneret has been followed by SEM of fixed material from actively secreting nidamental glands. The extruded ribbon showed a predominantly biaxial fibril orientation in low angle x-ray diffraction patterns recorded with the beam passing horizontally through the marginal rib. This x-ray pattern is derived from a remarkably regular parabolic arrangement of fibrils superficially resembling that seen in biological twisted nematic liquid crystals. However, evidence is presented here that the arrangement in the marginal rib is novel, apparently arising from authentically curved fibres showing a splayed- or bent- rather than twisted-nematic construction. Evidence is also presented that the spinnerets are able to control molecular orientations in a nematic liquid crystal to produce this and the other arrangements seen in the egg case.