Collagen: An Analysis of Phylogenetic Aspects

Fine structure and molecular organization of the periostracum in a gastropod mollusc Buccinum undatum L. and its relation to similar structural protein systems in other invertebrates

The horny layer of periostracum which covers the shell of Buccinum undatum L. has been studied by a combination of the fine structural techniques including high resolution transmission electron microscopy and scanning electron microscopy as well as by chemical analysis and X-ray diffraction. It has been found that the main structural component is a tectin type protein with globular and probably coiled-coil α-helical regions accompanied by a small amount of polysaccharide. Much of the periostracum is built up of protein sheets superposed in a regular manner and stabilized by some type of covalent cross-linking involving aromatic molecules. The protein is one of the class of structural macromolecules called scleroproteins. Each sheet of protein is made up of molecular sub-units which have a characteristic dumb-bell shape and which are about 32 nm long and 6.5 nm wide at their globular ends. End-to-end long-axis aggregation of these units produces filaments which aggregate further by side-to-side association into ribbons and ultimately sheets. The side-to-side association is always in register and hence the sheets have a major transverse striation repeating at 32 nm intervals. The protein sheets can be ascribed a longitudinal axis in terms of the direction of their component filaments. On this basis it can be shown that successive superposed sheets are rotated in a horizontal plane through an angle of 20-25° relative to one another, in a constant direction either clockwise or anticlockwise. Such helicoidal organization is of the cholesteric liquid crystal type which is often found in a biological context, e.g. chitin fibril disposition in arthropod cuticle. This helicoidal layering of the protein sheets is manifested in oblique sections of periostracum as repeated parabolic lamellae. Irregularities in the form of the parabolic lamellae can be accounted for on the basis of the curvature and extensive folding of the periostracum. The outer and innermost layers of the periostracum tend not to show helicoidal organization but exhibit a different aggregation mode of the dumb-bell-shaped units into a three-dimensional hexagonally packed network matrix. This matrix is much interrupted by vacuoles and localized smooth transitions into the ribbon mode of aggregation. This ability to exist in both fibrous and network aggregation states is comparable to that known among the collagens and muscle proteins. The amino acid compositions and conformations of proteins which can form cholesteric helicoidal systems are reviewed and compared with the protein of Buccinum periostracum. This property is apparently confined to alpha helical rod-shaped proteins and globular tektins. The beta conformation does not favour cholesteric organization. The structures and compositions of other molluscan periostraca and periostracum- like structures from other invertebrate phyla are compared with the periostracum of Buccinum . While all periostraca and functionally related structures have certain basic features in common there is a considerable degree of variation at the molecular and organizational levels.

Collagen gene construction and evolution

Collagen genes appear to have been assembled by the tandem repetition of homologous primary (9 base pair), secondary (54 base pair), and tertiary (702 base pair) modules. In vertebrate interstitial collagen genes many of the secondary modules are separated by introns, but in invertebrate collagen genes the non-coding sequences lie near the ends of supposed tertiary modules and are therefore about 702 (54 X 13) base pairs apart. The genes for vertebrate interstitial collagens (types I-III) seem to have been constructed by the tandem repetition of five tertiary modules, three of which were subsequently shortened by internal deletions. This shortening of the gene resulted in the non-integral relationship between the period of the fibrils and the length of the molecules of vertebrate collagens, and was therefore responsible for the mechanical properties of the completed product. Comparisons of the amino acid sequences of various collagens indicate that the main types of collagen evolved about 800-900 million years ago, a date that agrees well with the fossil record of primitive Metazoa.

[Biochemical and biophysical characterization of collagen in Tenebrio molitor L. (Coleoptera, Tenebrionidae)]

The collagen from the mesenteric sheath of the tenebrionid insect Tenebrio molitor was extracted by limited pepsin digestion and purified. This collagen was characterized using CM-cellulose chromatography, sodium-dodecylsulfate disc-gel electrophoresis and aminoacid analysis. This molecule was found to be assembled from three identical alpha chains and could be represented by the formula (alpha) 3. The amino acid composition is characteristic of collagen (one-third glycine, high iminoacid content), with high content of hydroxylysine and low content of alanine. Cyanogen bromide digests of these chains indicated that they are not related to any of the known invertebrate or vertebrate chains of interstitial collagens. The molecular weight (M = 280000D) and length (290 nm) were typical, and the banding patterns of the segment-long-spacing crystallites (SLS) and of the reconstitued fibrils were very similar to type I collagen. The denaturation temperature (Td) was 30.7 degrees C and correlated with the total pyrrolidine content as observed in other collagens (von Hippel & Wong's relation). It was concluded that the collagen from this insect showed the classical biochemical and biophysical features of other invertebrate interstitial "primitive" collagens.

Cockroach collagen: isolation, biochemical and biophysical characterization

Collagen fibrils from the mesenteric connective sheath of the adult cockroach Periplaneta americana were extracted by enzymatic digestion with pepsin and were purified. Chromatographic studies and sodium dodecylsulfate electrophoresis revealed the presence of a single chain. It was demonstrated that the structure of this collagen could be represented by the formula (alpha)3. The amino acid composition is typical of collagens (one-third glycine, and a high imino acid content) and similar to that of type II. The carbohydrate content was high (8.8%), and the cyanogen bromide pattern was different from that of known collagens. The chains were linked by the stable intermolecular bond dihydroxylysinonorleucine. The banding patterns of the segment-long-spacing crystallites and of the reconstituted fibrils were similar to type I collagen. The molecular weight (Mr 280,000) and length (285 nm) were typical, but the denaturation temperature was high (38.5 degrees C). It was concluded that cockroach mesenteric collagen showed the characteristic features of invertebrate mesodermal collagens, except that of the thermal stability of the triple-helical structure.

Some observations on the fine structure of elastoidin

Electron microscopy of positively stained elastoidin reveals a fine struc­ture apparently identical with that of the collagen of rat tail tendon. The negatively stained pattern is clearly related to that of the collagen but differs from it; it resembles that of artificially cross-linked collagens more. X-ray diffraction fails to find evidence of any long range lateral order in the arrangement of molecules within the elastoidin. The interest­ing splitting of the meridional pattern upon drying is re-reported and its possible origins discussed. Some discussion of fibrillogenesis is presented based on a comparison of rat tail tendon collagen and elastoidin.

Fine structure of the dogfish egg case: a unique collagenous material

The fine structure of the dogfish egg case is described with special reference to the highly ordered, unique, collagen-containing fibrils. The outer layer of the case wall contains densely packed, amorphous granules, rich in tyrosine while approximately 98% of the thickness of the case is built up from orthogonally stacked laminae of closely packed, collagen-containing fibrils. These fibrils show a paracrystalline three-dimensional construction. A model for the structure of the B band of the fibril is proposed, based on appearances in transverse sections of different thickness and on two projection seen in longitudinal sections. The transverse projection of the unit cell appears to be a square lattice with sides approximately 110 A possibly containing a pseudocell with sides (see article). The structure of these fibrils is discussed in relation to those of rat tail tendon collagen.