Morphology of the Adhesive System in the Sandcastle Worm, Phragmatopoma californica

Abstract

The marine Sandcastle worm (P. californica) and related species live in composite mineralized tubes for shelter. They gather the mineral phase for free from the environment as sand grains and seashell bits with a crown of ciliated tentacles. The captured mineral particles are conveyed for inspection to the building organ — a pincer-shaped pair of dexterous palps in front of the mouth (Fig. 10.1). A dab of proteinaceous adhesive (Jensen and Morse, 1988) is secreted from the building organ onto suitable particles as they are pressed onto the end of the tube. The major protein components of the adhesive are a group of heterogeneous proteins, referred to as Pc3x, characterized by serial runs of 10–14 serine residues punctuated with single tyrosine residues (Zhao et al., 2005). Phosphorylation of more than 90% of the serines (Stewart et al., 2004) makes the Pc3 proteins polyacidic (pI<3). Other potential protein components identified biochemically (Waite et al., 1992) and by sequencing random cDNAs from an adhesive gland library (Endrizzi and Stewart, 2009) are generally polybasic with predicted pIs greater than 9. Amino acid analysis of secreted glue revealed that, in total, close to 50% of the adhesive protein residues are charged when serine phosphorylation is taken into account. The adhesive also contains Mg²⁺ and Ca²⁺ and a large fraction of the tyrosines are post-translationally hydroxylated to form 3,4-dihydroxyphenylalanine (DOPA), a residue shared with the adhesive plaque proteins of the mussel (Waite and Tanzer, 1981). Phosphates and o-dihydroxyphenols are well-known adhesion promoters.