Alternative splicing of the Endo16 transcript produces differentially expressed mRNAs during sea urchin gastrulation.

The Endo16 gene codes for an RGD-containing calcium-binding protein that is found on the basal surfaces and in the extracellular matrix of cells of the invaginating archenteron during sea urchin gastrulation. Previously, we have shown that Endo16 is a single copy gene and we have determined the coding sequence and analyzed the temporal and spatial expression of a 6.6-kb mRNA. In this report we demonstrate that two additional longer Endo16 mRNAs are produced by differential splicing rather than alternative promoter usage. cDNA clones for two 8.5-kb mRNAs have been isolated and analyzed. The two 8.5-kb mRNAs are identical to each other in the coding region and differ only in their 3' UTRs. The extended open reading frame of the 8.5-kb mRNAs code for domains already identified in the 6.6-kb mRNA, including two different types of calcium-binding motifs and a region with a highly conserved cysteine pattern similar to that found in Ecm1 in the mouse. The 6.6- and 8.5-kb mRNAs show overlapping but distinct temporal as well as spatial expression patterns during gastrulation.