A DROSOPHILA RECEPTOR TYROSINE PHOSPHATASE EXPRESSED IN THE EMBRYONICCNS AND LARVAL OPTIC LOBES IS A MEMBER OF THE SET OF PROTEINS BEARINGTHE HRP CARBOHYDRATE EPITOPE

Recent studies have defined several cell surface glycoproteins express ed in the developing nervous system of insect embryos that may be invo lved in axon outgrowth and guidance processes. These glycoproteins inc lude the fasciclins and a group of receptor-linked protein tyrosine ph osphatases (R-PTPs). In embryos, the fasciclins are localized to axona l subsets, while the R-PTPs appear to be expressed on most or all CNS axons. To identify other neuronal cell surface glycoproteins in the Dr osophila embryo, we have taken a biochemical approach. This is based o n the observation that antisera against horseradish peroxidase (HRP) r ecognize a carbohydrate epitope that is selectively expressed in the i nsect nervous system. A large number of neuronal glycoproteins (denote d ''HRP proteins'') apparently bear the HRP carbohydrate epitope. We h ave used polyclonal anti-HRP antibodies to purify these proteins from Drosophila embryos, and have obtained protein sequences from seven HRP protein bands. These data define three major HRP proteins as neurotac tin, fasciclin I, and an R-PTP, DPTP69D. Western blotting data suggest that fasciclin II, neuroglian, DPTP10D, and DPTP99A are also HRP prot eins. We show that DPTP69D, like the previously characterized R-PTPs, is localized to CNS axons in the embryo. In third instar larvae, DPTP6 9D expression is restricted to subsets of neuronal processes in the br ain, ventral nerve cord, and eye disk. In the optic lobes, DPTP69D is localized to the neuropils of the lamina and medulla, and to an array of parallel thick bundles that may be the transmedullary fibers of the developing lobula complex.