Embryonic expression of a Drosophila src gene: alternate forms of the protein are expressed in segmental stripes and in the nervous system.

Expression of the Drosophila src-related gene, Dsrc28C has been investigated at the protein level using monoclonal antibodies. This analysis has revealed that the Dsrc28C gene encodes two protein forms: a 66-kD doublet predicted from the sequence of a cDNA clone and an additional 55-kD form. The 66-kD protein doublet is observed first at the cellular blastoderm stage and is not detectable in embryos after 12 hr of development. Expression of the 55-kD protein lags behind that of the 66-kD doublet and then persists throughout embryogenesis. Indirect immunofluorescence microscopy reveals that Dsrc28C protein is localized to the cell periphery during cellular blastoderm and gastrulation. The cell periphery-associated staining is then resolved into ectodermal stripes along the fully extended germ band. After the stripes fade, cytoplasmic staining of the majority of cells within the central and peripheral nervous system is observed. The 66-kD protein was shown to represent the cell periphery-associated form of the protein through antibody staining of larval salivary glands expressing a heat shock promoter-driven, full-length Dsrc28C cDNA. Staining of embryos with a monoclonal antibody specific for the 66-kD protein indicates that the 55-kD protein is the nervous system form. Thus, the 66- and 55-kD proteins are products of the Dsrc28C gene, which exhibit different temporal and spatial patterns of expression in the embryo.