J3-crystallin of the jellyfish lens: Similarity to saposins

J3-crystallin, one of the three major eye-lens proteins of the cubomedusan jellyfish (Tripedalia cystophora), shows similarity to vertebrate saposins, which are multifunctional proteins that bridge lysosomal hydrolases to lip ids and activate enzyme activity. Sequence alignment of deduced J3-crystall in indicates two saposin-like motifs arranged in tandem, each containing si x cysteines characteristic of this protein family. The J3-crystallin cDNA e ncodes a putative precursor analogous to vertebrate prosaposins. The J3-cry stallin gene has seven exons, with exons 2-4 encoding the protein. Exon 3 e ncodes a circularly permutated saposin motif, called a swaposin, found in p lant aspartic proteases. J3-crystallin RNA was found in the cubomedusan len s, statocyst, in bands radiating from the pigmented region of the ocellus, in the tentacle tip by in situ hybridization, and in the embryo and larva b y reverse transcription-PCR. Our data suggest a crystallin role for the mul tifunctional saposin protein family in the jellyfish lens. This finding ext ends the gene sharing evolutionary strategy for lens crystallins to the cni darians and indicates that the putative primordial saposin/swaposin J3-crys tallin reflects both the chaperone and enzyme connections of the vertebrate crystallins.