Analysis of proteinaceous components of the organic matrix of endoskeletal sclerites from the alcyonarian Lobophytum crassum

Abstract

The mesoglea of alcyonarians is occupied by an abundance of minute calcitic sclerites. The sclerites of the alcyonarian Lobophytum crassum contain a water-soluble organic matrix comprising 0.48% of the sclerite weight and a water-insoluble fraction comprising 1.15% of the sclerite weight. Analysis of proteinaceous components in the soluble fraction shows a particularly high content of aspartic acid, followed by alanine, glycine, and glutamate. Aspartic acid, glycine, alanine, and glutamate are the most abundant residues in the insoluble fraction. In both cases, the fractions show the highest concentration of aspartic acid from the total proteins. In an in vitro assay, we show that the matrix proteins extracted from the calcitic sclerites induce the formation of amorphous calcium carbonate prior to its transformation into the calcitic crystalline form. We also show scanning electron micrographs of the rhombohedral calcite crystals used as template, the protein imprinted with these crystals. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of both matrices shows the protein fractions at 67 and 48 kDa. The soluble matrix shows two additional faint bands. Both fractions stain for a carbohydrate at 67 kDa, indicating a glycoprotein at this molecular weight. A newly derived protein sequence was subjected to bioinformatics analysis involving identification of similarities to other acidic proteins. The identification of these proteins in alcyonarian endoskeletal sclerites emphasizes the fundamental importance of such acidic proteins and sheds more light on the functions of these proteins in the processes of biocalcification.