Auxin-binding protein 1 (ABP1) is a unique hormone receptor because it resides primarily in the lumen of the endoplasmic reticulum (ER); however, two lines of evidence presented here suggest that ABP1 does not bind auxin within the endoplasmic reticulum, despite its predominant location there. First, ABP1 cannot be photolabeled in intact cells that have accumulated the auxin and photolabeling reagent 5-[7-H]azidoindole-3-acetic acid, indicating either that auxin is excluded from the ER and is not available for photolabeling to ABP1 or that binding conditions within the ER lumen are insufficient for photolabeling. Second, at the pH of the ER lumen, auxin binding to ABP1 is not detectable. The pH estimate of the ER lumen is based on an indirect assay, which indicates that the pH is closer to pH 7 than to the binding optimum of pH 5.5. These results indicate that ABP1 does not bind auxin within the ER and point to a site of action that is post-ER. The effect of auxin on its trafficking from the ER was tested in an animal expression system. ABP1 expressed at high levels in COS7 cells is efficiently retained in the ER lumen and is not secreted even in the presence of 190 µM indole-3-acetic acid, an auxin concentration that is 40 times above the K for indole-3-acetic acid binding to ABP1.

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