Auxins are central hormones in all plants and controls virtually all aspects of growth and development. The PIN-FORMED (PIN) protein family is a key player in this process. Here we present biophysical analysis and structures of Arabidopsis thaliana PIN8 at 2.9-3.4 Å resolution; two outward facing conformations with and without auxin bound, and one inward facing conformation with the known inhibitor and herbicide naphthylphthalamic acid (NPA) bound. The structure forms a homo-dimer with each monomer divided into a transport and scaffold domain with a clearly defined auxin binding site. Next to the binding site, a proline-proline crossover is a pivot point for structural changes associated with transport, which we show by biophysical analysis to be independent of proton and ion gradients and likely driven by the negative charge of the auxin. Our results provide the first comprehensive molecular model for auxin recognition and transport by PINs.