© 1977 by The Society for Integrative and Comparative Biology
Brachiopod Shell Proteins: Their Functions and Taxonomic Significance
Geology Department, The Queen's University of Belfast Belfast, Northern Ireland
The calcareous shell of the Brachiopoda is interspersed with organic material, chiefly protein and polysaccharide. The amino acid compositions of these proteins reflect their genetically coded biosynthesis and are phylogenetically and taxonomically informative. The taxonomic scheme based on protein data agrees with the scheme based on morphological and anatomical criteria. These findings indicate Crania occupies an anomolous position.
Brachiopoda exhibit two main types of calcification, carbonate and phosphate. The hydroxyproline found in phosphatic inarticulate shell protein suggests an analogy with bone collagen, but the glycine content is too low to allow triple-helix formation.
The number and nature of polypeptide chains making up the shell proteins have been determined by amino and carboxy end-group analysis as well as disc electrophoresis with SDS. In the native state the shell proteins are molecular aggregates and are dissolved by 8 M urea, suggesting that the inter-chain links are largely H–bonds.
Articulate shell protein probably serves as a resilient cushioning between calcite fibers to protect against mechanical shock. This would be permitted by the amorphous flexible character of the polypeptide chain. The shell proteins of the Inarticulata are different,their chitin-protein laminated shell is more sheet-like and its structure requires less cushioning.
Study of fossil protein can shed further light on shell protein ancestry and hence on brachiopod phylogeny.