Cloning and characterization of novel beta integrin subunits from a sea urchin

Cell surface molecules that mediate adhesion in sea urchin embryos have been implicated in morphogenesis, and yet the molecules remain largely uncharacterized. Here we report evidence from PCR amplification for three novel beta integrin subunits that are expressed during early development of Strongylocentrotus purpuratus. The full cDNA sequence for one of these, betaG, bears a 59% similarity to Drosophila betaPS and a 58% similarity to vertebrate integrins. betaG closely resembles the beta1 subunit of vertebrates, particularly in the cytoplasmic domain where amino acids of the human beta1 subunit implicated in cell adhesion and signaling are conserved. The betaG subunit is detectable as a maternal, 7.5-kb transcript in eggs and expression peaks during gastrulation. Immunoblots with antiserum raised against a bacterially expressed fragment of the betaG subunit have bands with apparent molecular weights of about 130 kDa under reducing conditions and 110 kDa under nonreducing conditions. Immunoprecipitations suggest that betaG associates with at least two alpha subunits in gastrula stage embryos. In situ RNA hybridization of the betaG subunit indicates that all cells of the embryo express this molecule prior to gastrulation. In gastrulae, hybridization of the probe is highest in primary mesenchyme, secondary mesenchyme, the developing gut, and pigment cells. In immunolocalizations all cells of the blastulae express the protein at low levels and primary mesenchyme cells express betaG after they enter the blastocoel. Expression of the protein appears to be downregulated in the archenteron throughout gastrulation. betaG protein expression is also evident on secondary mesenchyme as they ingress and migrate in the blastocoel. We conclude that sea urchin embryos express integrins that are structurally similar to those characterized in other animals. Because betaG is expressed by migrating mesenchyme and yet is downregulated by rearranging epithelia, we suggest that this subunit has several functions during early development.