In sensory neurons, successful maturation of signaling molecules and regulation of Ca2+ are essential for cell function and survival. Here, we demonstrate a multifunctional role for calnexin as both a molecular chaperone uniquely required for rhodopsin maturation and a regulator of Ca2+ that enters photoreceptor cells during light stimulation. Mutations in Drosophila calnexin lead to severe defects in rhodopsin (Rh1) expression, whereas other photoreceptor cell proteins are expressed normally. Mutations in calnexin also impair the ability of photoreceptor cells to control cytosolic Ca2+ levels following activation of the light-sensitive TRP channels. Finally, mutations in calnexin lead to retinal degeneration that is enhanced by light, suggesting that calnexin's function as a Ca2+ buffer is important for photoreceptor cell survival. Our results illustrate a critical role for calnexin in Rh1 maturation and Ca2+ regulation and provide genetic evidence that defects in calnexin lead to retinal degeneration.

Calnexin is a molecular chaperone and a component of the quality control of the secretory pathway. We have generated calnexin gene-deficient mice (cnx ؊/؊ ) and showed that calnexin deficiency leads to myelinopathy. Calnexin-deficient mice were viable with no discernible effects on other systems, including immune function, and instead they demonstrated dysmyelination as documented by reduced conductive velocity of nerve fibers and electron microscopy analysis of sciatic nerve and spinal cord. Myelin of the peripheral and central nervous systems of cnx ؊/؊ mice was disorganized and decompacted. There were no abnormalities in neuronal growth, no loss of neuronal fibers, and no change in fictive locomotor pattern in the absence of calnexin. This work reveals a previously unrecognized and important function of calnexin in myelination and provides new insights into the mechanisms responsible for myelin diseases.The endoplasmic reticulum (ER) 5 is the first compartment in the secretory pathway responsible for protein synthesis, posttranslational modification, and correct folding. The resident molecular chaperones ensure that only correctly folded proteins leave the ER. Calnexin is a type I ER membrane protein, a major component in assuring the quality control of the secretory pathway, and together with calreticulin and the oxidoreductase ERp57, it promotes the correct folding of newly synthesized glycoproteins (2). Calnexin and calreticulin bind monoglucosylated carbohydrate on newly synthesized glycoproteins, whereas ERp57 catalyzes rearrangements of disulfide bonds within the calnexin/calreticulin substrate proteins (2). Despite its ubiquitous expression, the absence of calnexin has a different effect in different organisms. Calnexin deficiency is lethal in Schizosaccharomyces pombe but not in Saccharomyces cerevisiae (3), Dictyostelium (4, 5), or Caenorhabditis elegans (6, 7). The loss of calnexin affects phagocytosis in Dictyostelium (4, 5) and promotes necrotic cell death in C. elegans (7). It has been reported that deletion of the calnexin gene in a mouse results in early postnatal death (1), and thus the molecular consequences of calnexin deficiency could not be studied.Here we show that calnexin deficiency in the mouse did not result in early postnatal death (1). These animals developed myelinopathy with no discernible effects on other systems, including immune function. The phenotype was linked to slow nerve conduction velocities in the absence of calnexin with evidence of peripheral axon dysmyelination. The dysmyelinating phenotype described here underscores the emerging importance of calnexin and ER-associated pathways as contributors to these severe neurological disorders.

SUMMARY TRP channels have emerged as key biological sensors in vision, taste, olfaction, hearing and touch. Despite their importance, virtually nothing is known about the folding and transport of TRP channels during biosynthesis. Here, we identify XPORT (exit protein of rhodopsin and TRP) as a critical chaperone for TRP and its G-protein coupled receptor (GPCR), rhodopsin (Rh1). XPORT is a resident ER and secretory pathway protein that interacts with TRP and Rh1, as well as with Hsp27 and Hsp90. XPORT promotes the targeting of TRP to the membrane in Drosophila S2 cells, a finding that provides a critical first step towards solving a longstanding problem in the successful heterologous expression of TRP. Mutations in xport result in defective transport of TRP and Rh1, leading to retinal degeneration. Our results identify XPORT as a novel molecular chaperone and provide a mechanistic link between TRP channels and their GPCRs during biosynthesis and transport.