Exp. Eye Res. (1991) 53, 171-178 Identification of an S-antigen-like Molecule in Drosophila me/anogaster: An lmmunohistochemical Study WOLFGANG E. LIEB”, LAURA SMITH-LANG”, HARMINDER SINGH DUA”, ALAN C. CHRISTENSENb~~~ LARRY A. DONOSOa* a The Retina Service, Wills Eye Hospital, and b Department of Biochemistry and Molecular Biology, Jefferson Medical College, Thomas Jefferson University, Philadelphia, PA, and “Alcon Laboratories, Fort Worth, TX, U.S.A. (Received 8 June 7990 and accepted in revised form 26 November 1990) Vision is a highly complex process common to many vertebrate and invertebrate species.The visual system of Drosophila represents a valuable model in which to study this process since its chromosomal makeup has been well mapped and many mutants containing specific photoreceptor defectshave been described. S-antigen is a well-characterized photoreceptor cell protein which plays a fundamental role in the visual process. In this study, we used a panel of four monoclonal antibodies (MAbs) specific for amino, mid and carboxy terminal amino acid sequences in S-antigen in order to examine S-antigen immunoreactivity in the Drosophila visual system. Two MAbs, MAbSC6.47 and MAbClO-ClO,localized S-antigen to the retinules and ocelli. In contrast, MAbA2-G5 and MAbA9-C6 did not stain. The presence and strong antigenic similarity of an S-antigen homolog in Drosophilasuggeststhat in both vertebrates and invertebrates S-antigen plays a major role in the regulation of the phototransduction of vision. Key words: compound eye: Drosophilamelunogaster : immunohistochemistry : monoclonal antibody ; S- antigen : vision. 1. Introduction Vision is a fundamental physiological process common to many vertebrate and invertebrate species. Although many differences exist among these visual systems, recent studies indicate that the initial events in visual excitation may be quite similar. For example, the compound eye of Drosophila consists of a hexagonal array of approximately 800 facets or ommatidia, each containing eight photoreceptor cells which convert light signals into nerve impulses (see Fig. 1) (Tom- linson, 1988: Banerjee and Zipursky, 1990). Despite these obvious anatomical differencesfrom the human eye, in both systems light appears to activate the photopigment rhodopsin which in turn initiates a cascade of events eventually resulting in vision. Therefore, the identification and characterization of those molecules which participate in the visual system of Drosophila may provide important information regarding the visual process in higher vertebrates including humans. S-antigen is a highly conserved photoreceptor cell protein which plays a fundamental role in the visual process (Wacker et al., 1977; Das et al., 1984; Shinohara et al., 1989). Although its exact role in vision is unknown it is believed to have a regulatory role by competing with a-transducin for photoexcited and phosphorylated rhodopsin (Kiihn et al., 1984 ; Pfister et al., 19 8 5 ). In addition to its role in vision, S- antigen is highly pathogenic for the induction of experimental autoimmune uveitis (EAU), a predom- inantly T-cell mediated inflammatory disease which is characterized in its later stages by the complete destruction of the photoreceptor cells of the eye. S-antigen is also a highly antigenic molecule in which the immune response is generated almost exclusively towards the carboxyl terminal portion of the molecule (Donoso et al., 1986 ; Banga et al., 1987a, c; Donoso et al., 1988; Knospe et al., 1989). Recently, however, using several different immuniz- ation techniques, it has been possible to prepare monoclonal antibodies (MAbs) against bovine S- antigen which span the entire length of the molecule (Donoso et al., 1985b, 1990). These and other MAbs identify S-antigen in a variety of vertebrate and invertebrate species (Mirshahi et al., 1984, 1985; Van Veen et al., 1986; Banga et al., 1987b, 1988). Recently, two Drosophila genes encoding as S- antigen-like molecule have now been identified (Hyde et al., 1990; Smith et al., 1990; Yamada et al., 1990). Analysis of their amino acid sequence indicates an extensive sequence similarity with mammalian S- antigen. Based on the amino acid sequence of Drosophila S-antigen and the known binding sites of the MAbs, we characterized immunohistochemically an S-antigen-like molecule in the Drosophila eye. The significance of this finding with regard to the human visual system is discussed. * For reprint requests at: ResearchDepartment, Wills Eye Hospital,9th and Walnut Streets, Philadelphia, PA 19107, U.S.A. 0014-4835/91/080171+08 $03.00/O 0 1991 Academic Press Limited