The protein of brachiopod shell—I. Amino acid composition and implied protein taxonomy

Abstract

1. 1. The shell proteins of the Brachiopoda are being studied for the phylogenetic information they may carry in their amino acid composition. Eleven species have so far been examined. 2. 2. Amino acid compositions of these brachiopod shell protein webs show the basis for a structural-protein taxonomy well related to the biological classification. They clarify this in some respects, drawing distinctions between orders, marking off some families, and emphasizing the anomalous position of the Craniacea, placed biologically among the Inarticulata but having carbonate and not phosphatic shell. 3. 3. These shell proteins are generally characterized by high glycine, aspartic 4. acid and proline values. Those of the phosphatic inarticulates have also high alanine and are unusual among shell proteins generally in having hydroxyproline, normally associated with phosphatic calcification as with collagen in bone. Of particular significance is the hydroxyproline in Lingula, a genus with a demonstrably long ancestry from the Ordovician (4·5 × 10 8 yr), the time of emergence of bony structures generally. The order Lingulida could carry phosphatic calcification and proline hydroxylases back to the Lower Cambrian. 5. 4. Disc electrophoresis in urea-acrylamide gels shows the protein webs of Crania to consist of one chief component entering the small-pore gel and higher polymers not entering it.