Functional and structural analysis of Drosophila neuroglian

Drosophila neuroglian, an integral membrane glycoprotein, mediates cell adhesion via homophilic interactions. The extracellular region of the molecule consists of six immunoglobulin (Ig) domains followed by five fibronectin (Fn) domains. Neuroglian has two natural forms which are identical in the extracellular region but differ in the cytoplasmic domains. The expression of one form is neural specific while the other is present on a wide range of cells and tissues. To determine the effect of cytoplasmic domains on cell adhesion, both neuroglian cDNAs were transfected into and expressed in non-adhesive Drosophila S2 cells. The result showed that both forms mediated cell adhesion and that the two molecules recognized each other. A third artificial neuroglian form was constructed where the entire transmembrane and cytoplasmic domains of neuroglian were deleted leaving only a phosphoinositol moiety to anchor the protein in the lipid-bilayer. This protein not only showed cell adhesive property but also recognized the two natural forms of neuroglian. This result indicates that neither the transmembrane nor the cytoplasmic domain of neuroglian is involved in cell adhesion. To determine which functional domains in the extracellular region of neuroglian are involved in cell adhesion, specific domains of the molecule were deleted using recombinant DNA techniques. Variant forms of neuroglian were tested for function by the same cell aggregation assay. The data indicated that removal of either the first or the fourth immunoglobulin domain abolished cell adhesion, and strikingly, the first Ig domain by itself was sufficient for cell adhesion. The fifth and sixth immunoglobulin domains along with the fibronectin domains played little or no roles in homophilic cell adhesion. With the data presented in this study, I propose two possible models for the homophilic cell adhesion of neuroglian