Alternate splicing may generate a fly immune receptor

Fruit flies have a large protein called Dscam (Down syndrome cell adhesion molecule) with 24 exons. Four of these exons are alternatively spliced from 12, 48, 33, and 2 variants, potentially generating over 18,000 slightly different Dscam proteins. The protein is expressed by brain cells and cells involved with protection against infectious agents: called fat bodies and hemocytes. Dscam is found on the surface of these cells and the extracellular portion of the protein, clearved from the cytoplasmic portion, is found in the insect equivalent of blood, called the hemolymph. The function of Dscam was not known, but Schmucker and colleagues suspected a role in immune protection. They found that many of the gene variants are expressed in the fat bodies and hemocytes, though even more variants are expressed in the brain. Most strikingly, reducing expression of Dscam with RNA inhibition or using null mutants reduces the phagocytosis of bacteria by hemocytes. Two Dscam variants, but not a third, bind one strain of bacteria, supporting the notion that these variants might protect against different pathogens. Moreover, the alternative splicing of Dscam is conserved in other insects. Although the human homologue is not alternatively spliced, might there be other Dscam-like receptors, analogous to Toll-like-receptors (TLRs) in the mammalian genome?"
Watson et al., Science. 2005 Sep 16 309(5742):1874