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Structural and Functional Implications of the Instability of the ADP/ATP Transporter Purified from Mitochondria as Revealed by FTIR Spectroscopy

Víctor A. Lórenz-Fonfría ^*, Joaquim Villaverde ^*, Véronique Trézéguet , Guy J.-M. Lauquin , Gérard Brandolin  and Esteve Padrós ^*

^* Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, Universitat Autònoma de Barcelona, Spain; Laboratoire de Physiologie Moléculaire et Cellulaire, Institut de Biochimie et Génétique Cellulaires du Centre Nationale de la Recherche Scientifique, Bordeaux, France; and Laboratoire de Biochimie et Biophysique des Systèmes Intégrés, UMR 5092 CEA-CNRS-UJF, Département de Réponse et Dynamique Cellulaires, Commisariat à l'Energie Atomique, Grenoble, France

Correspondence: Address reprint requests to Esteve Padrós, Unitat de Biofísica, Facultat de Medicina, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain. Tel.: 34-93-581-1870; Fax: 34-93-581-1907; E-mail: esteve.padros{at}uab.es.

The ADP/ATP transporter shows a high instability when solubilized, making it difficult to obtain functional protein with sufficient purity for long-term spectroscopic studies. When solubilized in the detergent dodecyl maltoside the protein is in equilibrium between the so-called CATR and BA conformations and in a few hours it becomes nonfunctional, unable to bind either its inhibitors or its substrates. By Fourier transform infrared spectroscopy, we studied the structural changes involved in this denaturation process. To do so, the carboxyatractyloside-inhibited protein was used as a structural model for the protein in the CATR conformation and its spectrum was compared with that of the unliganded time-inactivated protein. From the difference spectra of the amide I, amide II, and amide A bands combined with dichroism spectra of the carboxyatractyloside-inhibited protein, we concluded that few structural differences exist between both states, affecting as few as 11 amino acids (3.5% of the protein); the structural changes consisted in the disappearance of large loop structure and the appearance of aggregated strands. We hypothesize that some mitochondrial loop (tentatively loop M1) shows a high tendency to aggregate, being responsible for the observed features. The functional consequences of this hypothesis are discussed.