Characterization of a putative Drosophila GTP-binding protein

We generated a set of monoclonal antibodies raised against Drosophila antigen eluted from a lectin affinity column. One antibody, mAb 13D5 recognizes an antigen found in the most dorsal regions of the ventral midline of Drosophila embryos at stages prior to and during axonogenesis. 13D5 recognizes cells dorsal to the ectoderm in the extended germ band beginning approximately 7 hours after fertilization and in the peripheral nervous system (PNS) at about 13 hours. In addition, 24-hour-old cultures of isolated embryonic neuroblasts possess a number of cells that express the 13D5 antigen and are not recognized by horseradish peroxidase antisera. These cells extend ramified processes with multiple growth cone-like structures or large individual processes with a broad growth cone structure. 13D5 immunoprecipitates a single band with an apparent molecular mass of 58 kDa and isolated a 1.9 kb EcoRI fragment from the lambda gt11 expression libraries. In situ hybridization to staged embryos using the digoxygenin-labeled probe reveals a pattern of expression in cells just lateral to the dorsal-most regions (mesectoderm) of the ventral midline in 8- to 9-hour embryos. In situ hybridization to cultured cells derived from whole embryos reveals several cell types with differing morphologies that express transcript recognized by the digoxygenin-labeled probe. These cells may possess either broad processes containing detectable transcript, or long thin processes with no detectable transcript. Northern analysis reveals a 2.1 kb RNA transcript detectable in all embryonic stages. Nucleotide sequence obtained from the 1.9 kb insert reveals homology with the GTP-binding regions of two signal recognition particle receptors (SNRP) isolated from canine and human tissues.