The Phosphatidylinositol Transfer Protein Domain of Drosophila Retinal Degeneration B Protein Is Essential for Photoreceptor Cell Survival and Recovery from Light Stimulation

Abstract

The Drosophila retinal degeneration B (rdgB) gene encodes an integral membrane protein involved in phototransduction and prevention of retinal degeneration. RdgB represents a nonclassical phosphatidylinositol transfer protein (PITP) as all other known PITPs are soluble polypeptides. Our data demonstrate roles for RdgB in proper termination of the phototransduction light response and dark recovery of the photoreceptor cells. Expression of RdgB's PITP domain as a soluble protein (RdgB-PITP) in rdgB2 mutant flies is sufficient to completely restore the wild-type electrophysiological light response and prevent the degeneration. However, introduction of the T59E mutation, which does not affect RdgB-PITP's phosphatidylinositol (PI) and phosphatidycholine (PC) transfer in vitro, into the soluble (RdgB-PITP-T59E) or full-length (RdgB-T59E) proteins eliminated rescue of retinal degeneration in rdgB2 flies, while the light response was partially maintained. Substitution of the rat brain PITPα, a classical PI transfer protein, for RdgB's PITP domain (PITPα or PITPα-RdgB chimeric protein) neither restored the light response nor maintained retinal integrity when expressed in rdgB2 flies. Therefore, the complete repertoire of essential RdgB functions resides in RdgB's PITP domain, but other PITPs possessing PI and/or PC transfer activity in vitro cannot supplant RdgB function in vivo. Expression of either RdgB-T59E or PITPα-RdgB in rdgB + flies produced a dominant retinal degeneration phenotype. Whereas RdgB-T59E functioned in a dominant manner to significantly reduce steady-state levels of rhodopsin, PITPα-RdgB was defective in the ability to recover from prolonged light stimulation and caused photoreceptor degeneration through an unknown mechanism. This in vivo analysis of PITP function in a metazoan system provides further insights into the links between PITP dysfunction and an inherited disease in a higher eukaryote.

Nonvesicular Lipid Transfer from the Endoplasmic Reticulum

“…This model was supported by both genetic and physiological studies on RdgB in Drosophila and its role in phototransduction cascade (Harris and Stark 1977;Vihtelic et al 1991Vihtelic et al , 1993Hardie et al 2001). Nevertheless, other studies suggest that PITPs are not involved in PI transport to the PM (Milligan et al 1997;Li et al 2000), and more recently, that PI could be delivered to the PM by specialized membrane compartments that carry PIS (Kim et al 2011). Studies on Sec14p, the major PITP in budding yeast, also suggest that it does not act as PI/PC-transfer protein in intact cells, but rather, as a scaffold that regulates the production of phosphoinositides and PC (Bankaitis et al 2010).…”

Phototransduction and retinal degeneration in Drosophila

“…6a) [28,29,[131][132][133]. In addition, a PI transfer protein (RDGB) may function in the transfer of PIs from the SRC to the rhabdomeres [29,134,135]. If the levels of CDS or PIS are reduced but not eliminated, a light response can still be obtained if the flies are dark-adapting [131,132].…”

Purification and cloning of phosphatidylinositol transfer proteins from Dictyostelium discoideum: homologues of both mammalian PITPs and Saccharomyces cerevisiae Sec14p are found in the same cell

“…In addition, the PITP domain has also been identified in the RdgB\Nir family, which are large proteins with putative membrane-spanning regions [2][3][4]. In Drosophila, RdgB functions together with rhodopsin, G q -related G-proteins and a phospholipase Cβ (PLCβ)-related enzyme (NorpA) in the Drosophila visual signal transduction system and was identified from a gene that causes retinal degeneration when mutated [5]. In mice, the ibrator mutation causes neurodegeneration via decreased expression of PITPα [6].…”