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(22) A FRESH LOOK AT UNCOUPLING PROTEINS M. Klingenberg, E. Winkler, K.
Echtay and K. Frischmuth
In the UCP subfamily UCP1 is the paradigm due to its ease of isolation from the native source. For other UCPs heterologous expression in yeast and E. coli was necessary to study their function. From UCP1 divergent characteristics, i.e. insensitivity to fatty acids and nucleotides are found for UCP2 and 3 in yeast mitochondria, cast doubt on whether UCP2 and 3 have an uncoupling function. At variance we showed that UCP3 expressed in yeast, different from UCP1 is not in a native but deranged state, and thus results with UCP3 yeast mitochondria were misleading. Reconstitution of UCP1, 2 and 3 expressed in E. coli allowed to recover highly nucleotide sensitive Cl- but no H+ transport, indicating that UCP2 and UCP interact with nucleotides. With rigorous renaturations conditions also nucleotide binding to soluble UCP2 and UCP3 could be recovered. In a long development with reconstituted UCP1 from E. coli now also H+ transport could be measured, which is highly sensitive to nucleotides (KI < 1µM), and dependent on fatty acids. Extrapolating these conditions subsequently also by H+ transport for UCP2 and UCP3 seem to follow the paradigm UCP1, in being fatty acid dependent and inhibited by nucleotide. Of importance for the physiological role of the UCP variants are differences in the ADP/ATP response: whereas UCP1 and UCP2 are stronger inhibited by ATP than ADP, UCP3 is more inhibited by ADP than ATP. This was rationalized with the opposite uncoupling requirements: UCP1 and obviously also UCP2 are active at a low ATP/ADP ratio, in line with preponderant energy dissipation by uncoupling, whereas UCP3 is active when ADP is low in the resting state of muscle at a high ATP/ADP ratio, thus supplementing uncoupling for thermogenes.
For further information contact...Carmen Mannella: carmen@wadsworth.org |
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