Receptor demise from alteration of glycosylation site in Drosophila opsin: electrophysiology, microspectrophotometry, and electron microscopy.

Abstract

In the delta Asn20 Drosophila stock, the N-linked glycosylation site of opsin in R1-6 receptors (Rh1) is absent. We used electroretinography (ERG), microspectrophotometry (MSP), and electron microscopy (EM) to quantify visual cell defects. Positive controls, w9, had wild type Rh1. MSP revealed minimal photopigment in delta Asn20 for 6 days posteclosion; w9 had near normal visual pigment. ERG sensitivity and prolonged depolarizing afterpotential (PDA) were compared for delta Asn20 and w9. Delta Asn20's R1-6 function is decreased 100-fold at eclosion and diminishes until only R7/8 functions at 11 days. What little rhodopsin is routed to the rhabdomere functions. Morphometry showed smaller R1-6 rhabdomeres in delta Asn20 for 8 days posteclosion. Rhabdomeres in w9 were normal. A negative control, ninaE(ol17), a deletion of the Rh1 gene, also has small rhabdomeres. Delta Asn20 and ninaE(ol17) lack the extreme rhabdomere elimination of ora (outer rhabdomeres absent), a nonsense mutant interrupting Rh1's coding sequence. Delta Asn20 and ora have surplus membrane while ninaE(ol17) does not. Freeze fracture reveals that delta Asn20's rhabdomeric P-face particle count is as low as for vitamin A deprivation, consistent with an opsin defect. High particle density, organized into rows, is present in adjacent plasmalemma where surplus membrane accumulates. In summary, delta Asn20 interferes with either synthesis, deployment, or maintenance of opsin.