![]() In rods, phototransduction is activated by the photoisomerization of the rhodopsin-bound chromophore 11- cis-retinal to all- trans retinal, inducing a conformational change in rhodopsin to its activated form metarhodopsin II. Phototransduction is a complex mechanism in which light is converted into an electrical signal through the sequential activation of signalling proteins. Rods and cones share the same cellular mechanism of light detection, a process known as phototransduction. ![]() Rhodopsin, with a peak absorption ( λ max) of ~500 nm, functions during dim light conditions allowing scotopic vision, whereas cone opsins are responsible for processing wavelengths ranging from ~350 to 560 nm, thus allowing colour vision. The rod OS contains the rod-specific photopigment rhodopsin, whereas the cone OS contains one of the three cone-opsins, S-opsin, M-opsin, or L-opsin. Opsins are responsible for tuning the absorption of light to a specific wavelength of the light spectrum. The OS of both cell types consists of closely spaced membranous discs containing photopigment molecules, called opsins, which are coupled to a light-absorbing chromophore (retinal, an aldehyde of vitamin A). Morphologically, photoreceptors consist of a synaptic terminal, a nuclear region, and an inner segment (IS) and outer segment (OS) which are connected by a connecting cilium (CC). Both rods and cones are adjacent to the retinal pigment epithelium (RPE), a monolayer of pigmented cells which is vital for the normal function and survival of photoreceptors. In the mammalian retina, there are two types of photoreceptor cells, the rods and the cones. Photoreceptor cells are highly specialized sensory neurons in the retina, and are essential for converting light into a neural signal, a fundamental process which initiates vision. Here, the interaction of Hsp90 with the retina-specific client proteins PDE6 and GRK1 will be further discussed, providing additional insights for the role of Hsp90 in retinal disease. In addition, several studies have shown that chemical manipulation of Hsp90 has significant consequences, both in healthy and degenerating retinae, and this can be partially attributed to the fact that Hsp90 interacts with important photoreceptor-associated client proteins. ![]() This review explores how the different isoforms of Hsp90, including the cytosolic Hsp90α/β, the mitochondrial TRAP1, and the ER-specific GRP94, are involved in the different proteostatic mechanisms of photoreceptors, and elaborates on Hsp90 function when retinal homeostasis is disturbed. Therefore, the viability of photoreceptors relies on mechanisms that ensure a well-balanced and functional proteome that maintains the protein homeostasis, or proteostasis, of the cell. Photoreceptors are sensitive neuronal cells with great metabolic demands, as they are responsible for carrying out visual phototransduction, a complex and multistep process that requires the exquisite coordination of a large number of signalling protein components.
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