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.