An auxin-binding protein is localized to the plasma membrane of maize coleoptile cells: identification by photoaffinity labeling and purification of a 23-kda polypeptide.

Plasma membrane vesicles were isolated from maize (Zea mays L.) coleoptile tissue by aqueous two-phase partitioning and assayed for homogeneity by the use of membrane-specific enzymatic assays. Using 5-azido-[7-3H]indole-3-acetic acid ([3H]N3IAA), we identified several IAA-binding proteins with molecular masses of 60 kDa (pm60), 58 kDa (pm58), and 23 kDa (pm23). Using Triton X-114, we were able to selectively extract pm23 from the plasma membrane. We show that auxins and functional analogues compete with [3H]N3IAA for binding to pm23. We found that PAB130, a polyclonal antibody raised against auxin-binding protein 1 (ABP-1), recognized ABP-1 as well as pm23. This suggests that pm23 shares common epitopes with ABP-1. In addition, we identified an auxin-binding protein with a molecular mass of 24 kDa (pm24), which was detected in microsomal but not in plasma membrane vesicle preparations. Like pm23 this protein was extracted from membrane vesicles with Triton X-114. We designed a purification scheme allowing simultaneous purification of pm23 and pm24. Homogeneous pm23 and pm24 were obtained from coleoptile extracts after 7000-fold purification.