We have isolated three classes of monoclonal antibodies against Drosophila cell-surface antigens that are expressed with positional specificity in imaginal discs. Comparison of immunofluorescence patterns with the wing-disc fate map reveals that expression of the antigens is not directly related to the specific type of cuticular structure that a cell will make upon differentiation but depends on the position of the cell in the undifferentiated disc epithelium. On mature wing discs, each class of position-specific (PS) antibody binds nonuniformly with respect to the dorso-ventral compartment boundary, with PS1 antibodies binding primarily to dorsal cells and PS2 antibodies, to ventral cells. Antibodies of the different PS classes extract similar but nonidentical sets of large glycoproteins from cell lysates, and antibodies of the most general class, PS3, recognize the PS1 and PS2 antigens in addition to PS3-specific components. Thus, the distributions and molecular characteristics of the PS antigens suggest that the molecules are structurally and functionally related to one another.Much of the adult Drosophila epidermis and some internal structures derive, at metamorphosis, from imaginal discs, which are present in larvae as convoluted inpocketings from the larval hypoderm. [Imaginal disc morphology and development are reviewed by Poodry (1).] In mature third-instar larvae each imaginal disc, although still largely undifferentiated, is stably determined to make a particular region of the adult (2, 3), and the cells within a particular part of a disc are specified to make a particular adult structure (4) (see Fig. 1). This precise specification of pattern within the disc appears to depend on intercellular interactions (6, 7). Clonal analysis has shown that discs are subdivided into compartments, defined by the observation that clones of genetically marked cells do not cross a compartment boundary into the territory of neighboring compartments (8). Genetic studies indicate that at least some of these compartments are units of gene action in the development of the epidermis (9, 10), although the relationship between compartments and the precise specification of pattern elements is not clear. We have studied the molecular basis of early disc development by using monoclonal antibodies to detect cell-surface antigens with heterogeneous distributions in larval discs before overt differentiation occurs. We previously described (5) the distribution of one such antigen, recognized by monoclonal antibody DK.1A4. Comparison of this antigen's distribution with a cell-lineage analysis of the mature wing imaginal disc (which makes the adult wing and associated mesothorax) showed that expression of the antigen correlated with the dorso-ventral lineage restriction (the border between dorsal and ventral compartments) in the epithelium. It was also clear that the expression of the antigen was not tightly linked to the type of adult structure that a disc cell was specified to differentiate, but seemed rather to depend on ...

Monoclonal antibodies have served to characterize neurotactin, a novel Drosophila protein for which a role in cell adhesion is postulated. Neurotactin is a transmembrane protein, as indicated by epitope mapping and amino acid sequence. Similarly to other cell adhesion molecules, neurotactin accumulates in parts of the membrane where neurotactin‐expressing cells contact each other. The protein is only detected during cell proliferation and differentiation, and it is found mainly in neural tissue and also in mesoderm and imaginal discs. Neurotactin has a large cytoplasmic domain rich in charged residues and an extracellular domain similar to cholinesterase that lacks the active site serine required for esterase activity. The extracellular domain also contains three copies of the tripeptide leucine‐arginine‐glutamate, a motif that forms the primary sequence of the adhesive site of vertebrate s‐laminin.

Neurotactin is a 135 kd membrane glycoprotein which consists of a core protein, with an apparent molecular weight of 120 kd, and of N‐linked oligosaccharides. In vivo, the protein can be phosphorylated in presence of radioactive orthophosphate. Neurotactin expression in the larval CNS and in primary embryonic cell cultures suggests that it behaves as a contact molecule between neurons or epithelial cells. Electron microscopy studies reveal that neurotactin is uniformly expressed along the areas of contacts between cells, without, however, being restricted to a particular type of junction. It putative adhesive properties have been tested by transfecting non adhesive Drosophila S2 cells with neurotactin cDNA. Heat shocked transfected cells do not aggregate, suggesting that neurotactin does not mediate homophilic cell adhesion. However, these transfected cells bind to a subpopulation of embryonic cells which probably possess a related ligand. The location at cellular junctions between specific neurons or epithelial cells, the heterophilic binding to a putative ligand and the ability to be phosphorylated are consistent with the suggestion that neurotactin functions as an adhesion molecule.

Neurotactin (Nrt), a Drosophila transmembrane glycoprotein which is expressed in neuronal and epithelial tissues during embryonic and larval stages, exhibits heterophilic adhesive properties. The extracellular domain is composed of a catalytically inactive cholinesterase‐like domain. A three‐dimensional model deduced from the crystal structure of Torpedo acetylcholinesterase (AChE) has been constructed for Nrt and suggests that its extracellular domain is composed of two sub‐domains organized around a gorge: an N‐terminal region, whose three‐dimensional structure is almost identical to that of Torpedo AChE, and a less conserved C‐terminal region. By using truncated Nrt molecules and a homotypic cell aggregation assay which involves a soluble ligand activity, it has been possible to show that the adhesive function is localized in the N‐terminal region of the extracellular domain comprised between His347 and His482. The C‐terminal region of the protein can be removed without impairing Nrt adhesive properties, suggesting that the two sub‐domains are structurally independent. Chimeric molecules in which the Nrt cholinesterase‐like domain has been replaced by homologous domains from Drosophila AChE, Torpedo AChE or Drosophila glutactin (Glt), share similar adhesive properties. These properties may require the presence of Nrt cytoplasmic and transmembrane domains since authentic Drosophila AChE does not behave as an adhesive molecule when transfected in S2 cells.