Fig. 1.

Specific binding of Na⁺ to the I. tartaricus c-ring. Representative ITC experiment in which the I. tartaricus (green) and S. platensis (yellow) c-rings are titrated with NaCl at pH 6.5. The reference titration of the buffer in the absence of either protein (gray) is also provided. The Inset shows the atomic structures of the ion-binding sites in the I. tartaricus and S. platensis c-rings, in the Na⁺- and H⁺-bound states, respectively (21, 23, 49). A sequence alignment of the c-subunits in these two organisms highlights the location of all Asp/Glu side chains, including those in the functional ion-binding sites (*).