RECEPTOR DEMISE FROM ALTERATION OF GLYCOSYLATION SITE IN DROSOPHILA OPSIN - ELECTROPHYSIOLOGY, MICROSPECTROPHOTOMETRY, AND ELECTRON-MICROSCOPY

In the Delta Asn(20) Drosophila stock, the N-linked glycosylation site of opsin in R1-6 receptors (Rh1) is absent. We used electroretinograp hy (ERG), microspectrophotometry (MSP), and electron microscopy (EM) t o quantify visual cell defects. Positive controls, w9, had wild type R h1. MSP revealed minimal photopigment in Delta Asn(20) for 6 days post eclosion; w9 had near normal visual pigment. ERG sensitivity and prolo nged depolarizing after potential (PDA) were compared for Delta Asn(20 ) and w9. Delta Asn(20)'s R1-6 function is decreased 100-fold at eclos ion and diminishes until only R7/8 functions at 11 days. What little r hodopsin is routed to the rhabdomere functions. Morphometry showed sma ller R1-6 rhabdomeres in Delta Asn(20) for 8 days posteclosion. Rhabdo meres in w9 were normal. A negative control, ninaE(ol17), a deletion o f the Rh1 gene, also has small rhabdomeres. Delta Asn(20) and ninaE(ol 17) lack the extreme rhabdomere elimination of ora (outer rhabdomeres absent), a nonsense mutant interrupting Rh1's coding sequence. Delta A sn(20) and ora have surplus membrane while ninaE(ol17) does not. Freez e fracture reveals that Delta Asn(20)'s rhabdomeric P-face particle co unt is as low as for vitamin A deprivation, consistent with an opsin d efect. High particle density, organized into rows, is present in adjac ent plasmalemma where surplus membrane accumulates. In summary, Delta Asn(20) interferes with either synthesis, deployment, or maintenance o f opsin.