The backbone structure is determined by site-directed spin labeling, double electron electron resonance measurements of distances, and modeling in terms of a helix-loop-helix construct for a transmembrane domain that is supposed to line the translocation pathway in the 54.3 kDa Naþ/proline symporter PutP of Escherichia coli. The conformational distribution of the spin labels is accounted for by a rotamer library. An ensemble of backbone models with a root mean-square deviation of less than 2A˚ is obtained. These models exhibit a pronounced kink near residue T341, which is involved in substrate binding. The kink may be associated with a hinge that allows the protein to open and close an inwardly oriented cavity.