THE N-1-NAPHTHYLPHTHALAMIC ACID-BINDING PROTEIN IS AN INTEGRAL MEMBRANE-PROTEIN

N-1-Naphthylphthalmic acid (NPA)-binding protein is a plasmalemma (PM) protein involved in the control of cellular auxin efflux. We re-evalu ated the spatial relationship of this protein with the PM of zucchini (Cucurbita pepo L.) hypocotyls. First, Triton X-114 partitioning indic ated that the NPA-binding protein was more hydrophobic than most PM pr oteins. Second, the NPA-binding activity was found to be resistant to proteolytic digestion in membranes. Maximum concentrations of binding sites for NPA were virtually identical in untreated and proteinase K-t reated PMs: 19.2 and 20.6 pmol [H-3]NPA bound/mg protein, respectively . The insensitivity of the NPA-binding protein was not due to its pres ence inside tightly sealed vesicles or due to lack of protease activit y in the conditions tested. This protein could be made sensitive to pr oteolytic degradation upon solubilization by holamidopropyl)dimethylam monio]-1-propanesulfonate in the presence of sodium molybdate. Protein ase K treatment decreased the concentration of binding sites to 0.84 p mol [H-3]NPA bound/mg protein from 9.2 for untreated, solubilized PM. Third, this activity could not be solubilized by chaotropic agents or sodium carbonate treatment of intact PM. This study indicates that the NPA-binding protein may be an integral membrane protein and contradic ts previously reported findings that suggested that this protein was p eripheral to the PM.