Transducin Partners Outside the Phototransduction Pathway

Abstract

Transducin mediates signal transduction in a classical G protein-coupled receptor (GPCR) phototransduction cascade. Interactions of transducin with the receptor and the effector molecules had been extensively investigated and are currently defined at the atomic level. However, partners and functions of rod transducin α (Gα t 1 ) and βγ (Gβ 1 γ 1 ) outside the visual pathway are not well-understood. In particular, light-induced redistribution of rod transducin from the outer segment to the inner segment and synaptic terminal (IS/ST) allows Gα t1 and/or Gβ 1 γ 1 to modulate synaptic transmission from rods to rod bipolar cells (RBCs). Protein-protein interactions underlying this modulation are largely unknown. We discuss known interactors of transducin in the rod IS/ST compartment and potential pathways leading to the synaptic effects of light-dispersed Gα t1 and Gβ 1 γ 1 . Furthermore, we show that a prominent non-GPCR guanine nucleotide exchange factor (GEF) and a chaperone of Gα subunits, resistance to inhibitors of cholinesterase 8A (Ric-8A) protein, is expressed throughout the retina including photoreceptor cells. Recent structures of Ric-8A alone and in complexes with Gα subunits have illuminated the structural underpinnings of the Ric-8A activities. We generated a mouse model with conditional knockout of Ric-8A in rods in order to begin defining the functional roles of the protein in rod photoreceptors and the retina. Our analysis suggests that Ric-8A is not an obligate chaperone of Gα t1 . Further research is needed to investigate probable roles of Ric-8A as a GEF, trafficking chaperone, or a mediator of the synaptic effects of Gα t1 .

Frmpd1 Facilitates Trafficking of G-Protein Transducin and Modulates Synaptic Function in Rod Photoreceptors of Mammalian Retina

“…Currently, only a few interacting partners of Gα t have been identified outside of the phototransduction cascade, and some of these have functions related to the movement of Gα t between IS/OS ( Gopalakrishna et al, 2011 ; Zhang et al, 2011 ; Sinha et al, 2013 ; Chaya et al, 2019 ; Srivastava et al, 2020 ). Since Gα t translocates to the rod IS and synapse on bright light exposure where Frmpd1 and Gpsm2 are positioned, we wondered whether Frmpd1 and/or Gpsm2 could influence Gα t translocation dynamics between these subcellular compartments during dark adaptation in vivo .…”

“…Redistribution of key phototransduction components within photoreceptor compartments is critical for adaptation, survival, and effective synaptic signaling in response to varying light intensities. However, despite extensive investigations, the precise mechanism of light induced translocation remains poorly understood and controversial ( Nair et al, 2005b ; Peterson et al, 2005 ; Karan et al, 2008 ; Reidel et al, 2008 ; Kerov and Artemyev, 2011 ; Zhang et al, 2011 ; Majumder et al, 2013 ; Frederick et al, 2020 ; Srivastava et al, 2020 ). Here, we provide novel insights into the mechanisms of Gα t trafficking and synaptic transmission in rod photoreceptors.…”

“…The Avi tag of the His-Avi-Trx-Frmpd1-pept was replaced with an HA tag. The Avi-tagged Gα t * construct was described previously ( Srivastava et al, 2020 ). Proteins were expressed and purified similarly as described above.…”

“…Recent studies uncovered protein interactors involved in the unique trafficking and function of Gα t in rod IS and synapses ( Frederick et al, 2020 ; Srivastava et al, 2020 ). Gpsm2 (G-protein signaling modulator 2, LGN) localizes to these subcellular compartments and stabilizes inactive, GDP-bound Gα t in vitro ( Kerov et al, 2005a ; McCudden et al, 2005 ; Nair et al, 2005a ).…”

Light-dependent regulation of neurotransmitter release from rod photoreceptor ribbon synapses involves an interplay of Complexin 4 and Transducin with the SNARE complex

“…In addition to the SNARE complex proteins, our proteomic screen and flanking biochemical, imaging, and PLA data identified Gα t1 and Gβ 1 γ 1 as interactors of Cplx4-mediated SNARE complexes in vitro ( Figures 4 , 5 and Supplementary Data 2 ) and in vivo ( Figure 6 ). Beyond their role as key components of the phototransduction cascade in rod photoreceptor OS, the G t subunits Gα t1 and Gβ 1 γ 1 translocate to IS and synaptic terminals upon exposure to bright light ( Brann and Cohen, 1987 ; Sokolov et al, 2002 ; Arshavsky and Burns, 2012 ; Srivastava et al, 2020 ). This G t translocation is thought to reduce the phototransduction gain, thus contributing to light adaptation and reducing the metabolic stress associated with ongoing phototransduction under bright light ( Sokolov et al, 2002 ; Majumder et al, 2013 ; Frederiksen et al, 2021 ).…”