An α₂-Macroglobulin-Like Protein Is the Cue to Gregarious Settlement of the Barnacle Balanus amphitrite

• Published in: Proceedings of the National Academy of Sciences - PNAS 2006-09, Vol.103 (39), p.14396-14401
• Main Authors: Dreanno, Catherine, Matsumura, Kiyotaka, Dohmae, Naoshi, Takio, Koji, Hirota, Hiroshi, Kirby, Richard R., Clare, Anthony S.
• Format: Article
• Language: English
• Subjects: Amino acids; Biological Sciences; Complementary DNA; Glycoproteins; Insect larvae; Larvae; Messenger RNA; Nauplii; Polymerase chain reaction; Proteins; RNA
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0602763103

Many benthic marine invertebrates, like barnacles, have a planktonic larval stage whose primary purpose is dispersal. How these species colonize suitable substrata is fundamental to understanding their evolution, population biology, and wider community dynamics. Unlike larval dispersal, settlement occurs on a relatively small spatial scale and involves larval behavior in response to physical and chemical characteristics of the substratum. Biogenic chemical cues have been implicated in this process. Their identification, however, has proven challenging, no more so than for the chemical basis of barnacle gregariousness, which was first described >50 years ago. We now report that a biological cue to gregarious settlement, the settlement-inducing protein complex (SIPC), of the major fouling barnacle Balanus amphitrite is a previously undescribed glycoprotein. The SIPC shares a 30% sequence homology with the thioester-containing family of proteins that includes the α₂-macroglobulins. The cDNA (5.2 kb) of the SIPC encodes a protein precursor comprising 1,547 aa with a 17-residue signal peptide region. A number of structural characteristics and the absence of a thioester bond in the SIPC suggest that this molecule is a previously undescribed protein that may have evolved by duplication from an ancestral α⊂-macroglobulin gene. Although the SIPC is regarded as an adult cue that is recognized by the cyprid at settlement, it is also expressed in the juvenile and in larvae, where it may function in larva-larva settlement interactions.