Secondary-structure characterization by far-UV CD of highly purified uncoupling protein 1 expressed in yeast

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Abstract
The rat UCP1 (uncoupling protein 1) is a mitochondrial inner-membrane carrier involved in energy dissipation and heat production. We expressed UCP1 carrying a His6 epitope at its C-terminus in Saccharomyces cerevisiae mitochondria. The recombinant-tagged UCP1 was purified by immobilized metal-ion affinity chromatography to homogeneity (>95%). This made it suitable for subsequent biophysical characterization. Fluorescence resonance energy transfer experiments showed that n-dodecyl-β-d-maltoside-solubilized UCP1–His6 retained its PN (purine nucleotide)-binding capacity. The far-UV CD spectrum of the functional protein clearly indicated the predominance of α-helices in the UCP1 secondary structure. The UCP1 secondary structure exhibited an α-helical degree of approx. 68%, which is at least 25% higher than the previously reported estimations based on computational predictions. Moreover, the helical content remained unchanged in free and PN-loaded UCP1. A homology model of the first repeat of UCP1, built on the basis of X-ray-solved close parent, the ADP/ATP carrier, strengthened the CD experimental results. Our experimental and computational results indicate that (i) α-helices are the major component of UCP1 secondary structure; (ii) PN-binding mechanism does not involve significant secondary-structure rearrangement; and (iii) UCP1 shares similar secondary-structure characteristics with the ADP/ATP carrier, at least for the first repeat.