The structure and activities of echinonectin: a developmentally regulated cell adhesion glycoprotein with galactose-specific lectin activity

Abstract

The extracellular matrix of the sea urchin embryo contains a 230 kD homodimeric glycoprotein known as echinonectin (EN). EN contains a cell attachment domain as well as a galactose-specific lectin activity. Cell attachment to EN is differentially regulated in the three primary germ layers, endoderm, ectoderm and mesoderm. Prior to gastrulation all embryonic cells adhere equally to EN-coated substrates, but during gastrulation primary mesenchyme cells lose affinity for EN, ectoderm cells increase their binding to the molecule, and cells of the endoderm maintain a similar or slightly lowered level of binding. The mechanisms governing these adhesive changes and the specific functions they serve in development are not currently understood. They are timed to coincide with distinct morphogenetic events such as primary mesenchyme cell ingression and archenteron formation, suggesting that regulated adhesion to EN plays at least a permissive role in early morphogenesis.