Dr. Edmund R.S. Kunji, The Medical Research Council, Mitochondrial Biology Unit, Cambridge
Tuesday 1st December, 1.00 p.m., Keynes Lecture Theatre 6
Mitochondrial carriers transport nucleotides, amino acids, inorganic ions, fatty acids, keto acids, and cofactors across the mitochondrial inner membrane. They link the biochemical pathways in the cytoplasm and mitochondrial matrix, playing key roles in cellular metabolism and regulation. Here we report on the progress made in defining the structural mechanism of transport and regulation of mitochondrial carriers. We will combine bioinformatics, structural biology, biophysics and functional studies to describe the mechanism in molecular detail.
One of the members of the transport protein family, the mitochondrial ADP/ATP carrier, imports ADP from the cytosol and exports ATP from the mitochondrial matrix, replenishing the eukaryotic cell with metabolic energy. We have determined the atomic structures of two yeast mitochondrial ADP/ATP carriers in the cytoplasmic state. The carriers consist of three domains, each containing two transmembrane α-helices linked by a short matrix α-helix. They function according to an alternating access transport mechanism in which two salt bridge networks on matrix and cytoplasmic side of the central cavity regulate access to a central substrate binding site. Analyses of the domain structures and properties of the interdomain interfaces indicate that interconversion between states involves movement of the even-numbered α-helices across the surfaces of the odd-numbered α-helices by rotation of the domains. The simultaneous movement of three domains around a central translocation pathway constitutes a unique mechanism among transport proteins.
Another transport protein of the family, the mitochondrial aspartate/glutamate carrier imports glutamate and exports aspartate, and is central to human physiology, as it is involved in the aspartate-malate shuttle, urea cycle, myelin synthesis and gluconeogenesis. The carrier is a chimera, consisting of three separate domains; an N-terminal domain with eight EF-hands involved in calcium binding, a carrier domain involved in transport, and a C-terminal domain of unknown function. We have determined the structure of the regulatory domain in three different states, elucidating the molecular mechanism of calcium regulation of the mitochondrial aspartate/glutamate carrier.
The proposed transport and regulatory mechanism provide new insights into the molecular basis of several human diseases caused by dysfunctional mitochondrial carriers.