Cloning and Characterization of Novel β Integrin Subunits from a Sea Urchin

Abstract

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 β integrin subunits that are expressed during early development ofStrongylocentrotus purpuratus.The full cDNA sequence for one of these, βG, bears a 59% similarity toDrosophilaβPS and a 58% similarity to vertebrate integrins. βG closely resembles the β1 subunit of vertebrates, particularly in the cytoplasmic domain where amino acids of the human β1 subunit implicated in cell adhesion and signaling are conserved. The βG 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 βG subunit have bands with apparent molecular weights of about 130 kDa under reducing conditions and 110 kDa under nonreducing conditions. Immunoprecipitations suggest that βG associates with at least two α subunits in gastrula stage embryos.In situRNA hybridization of the βG 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 βG after they enter the blastocoel. Expression of the protein appears to be downregulated in the archenteron throughout gastrulation. βG 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 βG is expressed by migrating mesenchyme and yet is downregulated by rearranging epithelia, we suggest that this subunit has several functions during early development.