Direct evidence for the size and conformational variability of the pyruvate dehydrogenase complex revealed by three-dimensional electron microscopy - The "breathing" core and its functional relationship to protein dynamics

Structural studies by three-dimensional electron microscopy of the Saccharomyces cerevisiae truncated dihydrolipoamide acetyltransferase (tE(2)) component of the pyruvate dehydrogenase complex reveal an extraordinary example of protein dynamics. The tE(2) forms a 60-subunit core with the morphology of a pentagonal dodecahedron and consists of 20 cone-shaped trimers interconnected by 30 bridges. Frozen-hydrated and stained molecules of tE(2) in the same field vary in size similar to 20%, Analyses of the data show that the size distribution is bell-shaped, and there is an approximately 40-Angstrom difference in the diameter of the smallest and largest structures that corresponds to similar to 14 Angstrom of variation in the length of the bridge between interconnected trimers, Companion studies of mature E-2 show that the complex of the intact subunit exhibits a similar size variation, The x-ray structure of Bacillus stearothermophilus tE(2) shows that there is an similar to 10-Angstrom gap between adjacent trimers and that the trimers are interconnected by the potentially flexible C-terminal ends of two adjacent subunits. We propose that this springlike feature is involved in a thermally driven expansion and contraction of the core and, since it appears to be a common feature in the phylogeny of pyruvate dehydrogenase complexes, protein dynamics is an integral component of the function of these multienzyme complexes.