Retinol-binding protein and transthyretin expressed in HeLa cells form a complex in the endoplasmic reticulum in both the absence and the presence of retinol.
To establish a suitable experimental system for studies of the interaction of retinol-binding protein (RBP) with transthyretin (TTR) we have expressed the corresponding cDNAs in HeLa cells. To investigate whether complex formation might occur already in the endoplasmic reticulum (ER), the C-terminal ER retention signal, KDEL, was attached to TTR. The tetrameric TTR-KDEL fusion protein was retained in the ER of HeLa cells. When RBP was co-expressed with TTR-KDEL, RBP was retained intracellularly. A cDNA-encoding purpurin, a protein which is 50% identical to RBP, was then expressed together with TTR-KDEL. Purpurin was not retained intracellularly and did not bind to TTR coupled to Sepharose. The effect of the vitamin A status on the secretion of TTR and RBP was examined. While TTR expressed alone was not retained intracellularly, TTR was retained in vitamin A-deficient cells when co-expressed with RBP. Addition of retinol stimulated rapid secretion of both proteins. These results demonstrate that TTR can form a complex with RBP in the ER. The data suggest that RBP and TTR are secreted as a complex.
Department of Cell Research, Uppsala University, Sweden.
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Melhus
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Nilsson
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Peterson
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Rask
D000880:Anthraquinones; D001483:Base Sequence; D003001:Cloning, Molecular; D004247:DNA; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D037102:Lectins / Q000378:metabolism; D008969:Molecular Sequence Data; D011228:Prealbumin / Q000235:genetics / Q000378:metabolism*; D011233:Precipitin Tests; D011993:Recombinant Fusion Proteins / Q000235:genetics; D012177:Retinol-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D014801:Vitamin A / Q000378:metabolism*
D000880:Anthraquinones; D037102:Lectins; D011228:Prealbumin; D011993:Recombinant Fusion Proteins; D012177:Retinol-Binding Proteins; D014801:Vitamin A; D004247:DNA; C511975:purpurin anthraquinone