Cyclophilin-related protein RanBP2 acts as chaperone for red/green opsin

Abstract

Cyclophilins are ubiquitous and abundant proteins that exhibit peptidyl prolyl cis-trans isomerization (PPlase) activity in vitro. Their functions in vivo, however, are not well understood. Two new retinal cyclophilin isoforms, types I and II, are highly expressed in cone photoreceptors of the vertebrate retina. Type-II cyclophilin is identical to RanBP2, a large protein that binds the GTPase Ran. Here we report that two contiguous domains in RanBP2, Ran-binding domain 4 (RBD4) and cyclophilin, act in concert as a chaperone for the opsin molecule of the red/green-sensitive visual pigment of a dichromatic vertebrate. In Drosophila, the cyclophilin NinaA is expressed in all photoreceptors and is required for the expression of only a subset of opsins. The molecular basis of these photoreceptor class-specific effects and the functions of NinaA and other cyclophilins in vivo remain unclear. Unlike NinaA, which forms a stable complex with opsin from retinular cells R1-6, we find that the cyclophilin domain of RanBP2 does not bind opsin directly; rather, it augments and stabilizes the interaction between red/green (R/G) opsin and the RBD4 domain. This involves a cyclophilin-mediated modification of R/G opsin, possibly involving proline isomerization. The RBD4-cyclophilin supradomain of RanBP2, therefore, is a form of vertebrate chaperone of defined substrate specificity, which may be involved in the processing and/or transport of long-wavelength opsin in cone photoreceptor cells.

Cell-surface targeting of α2-adrenergic receptors — Inhibition by a transport deficient mutant through dimerization

“…We have demonstrated that, whereas the transport from the ER to the cell surface of AT1R and β 2 -AR is dependent on Rab1, α 2B -AR export is independent of Rab1, indicating that different G protein-coupled receptors may use distinct pathways for their transport from the ER to the cell surface [23]. We then identified a motif consisting of a phenylalanine and double leucine spaced by 6 residues [F(x) 6 LL] in the membrane-proximal carboxyl termini of α 2B -AR and AT1R, which is essential for the exit of the receptors from the ER [22]. The receptor mutants, α 2B -ARm and AT1Rm in which the F(x) 6 LL motif were mutated to alanines, were unable to transport to the cell surface and were retained in the ER.…”

“…We then identified a motif consisting of a phenylalanine and double leucine spaced by 6 residues [F(x) 6 LL] in the membrane-proximal carboxyl termini of α 2B -AR and AT1R, which is essential for the exit of the receptors from the ER [22]. The receptor mutants, α 2B -ARm and AT1Rm in which the F(x) 6 LL motif were mutated to alanines, were unable to transport to the cell surface and were retained in the ER. In this report, we sought to determine if α 2B -ARm could regulate the cell-surface targeting and signaling of its WT counterpart.…”

“…First, similar to many other membrane proteins, receptors must be correctly folded in order to pass the ER quality control mechanism [3,4]. Second, export from the ER and further transport to the cell surface of some G protein-coupled receptors requires specific accessory proteins or chaperones [5][6][7][8]. Interaction of the receptors with chaperones may regulate correct folding/assembly of the receptors within the ER and facilitates receptor transport from the ER and on to the cell surface.…”

“…We demonstrated that a phenylalanine and double leucine spaced by 6 residues [F(x) 6 LL] in the membrane-proximal carboxyl termini of α 2B -AR and angiotensin II type 1 receptor (AT1R) are required for exit from the ER [22]. Mutation of the F(x) 6 LL motif to alanines abolishes export of the receptors out of the ER and further transport to the cell surface.…”

“…Mutation of the F(x) 6 LL motif to alanines abolishes export of the receptors out of the ER and further transport to the cell surface. We report here that, similar to many other G protein-coupled receptors, α 2B -AR apparently homodimerizes in the ER, and that the α 2B -AR mutant deficient in ER export confers an inhibitory effect on the cell-surface targeting and function of its wild-type (WT) counterpart through heterodimerization in the ER.…”