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 ϳ20%. Analyses of the data show that the size distribution is bell-shaped, and there is an approximately 40-Å difference in the diameter of the smallest and largest structures that corresponds to ϳ14 Å 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 ϳ10-Å 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.The pyruvate dehydrogenase complexes (PDCs) 1 are among the largest (M r ϳ10 6 to 10 7 ) and most complex multienzyme structures known. A central feature of these complexes is a 24-mer (Escherichia coli) or 60-mer (eukaryotes and some Gram-positive bacteria) dihydrolipoamide acetyltransferase (E 2 ) core with the morphologies of a cube or a pentagonal dodecahedron, respectively (1-4). The cores have both functional and structural roles in organizing the multienzyme complex; the E 2 activity is associated with the scaffold to which the other components are attached. These include the pyruvate dehydrogenase (E 1 ) and dihydrolipoamide dehydrogenase (E 3 ), which requires a binding protein (BP) to anchor it to the core of the yeast and mammalian PDCs, although, in E. coli and Bacillus stearothermophilus PDCs, BP is not required (1-4).The E 2 subunits have multidomain structures consisting of one, two, or three amino-terminal lipoyl domains, followed by an E 1 and/or E 3 binding domain, and a carboxyl-terminal catalytic domain (1-4). X-ray crystallography (5-9) and threedimensional electron microscopy (10, 11) show that the E 2 catalytic domains are arranged in cone-shaped trimers at each of the 8 or 20 vertices of the cubic or dodecahedral structures, respectively (7,8,10,11). The trimers are interconnected by bridges to form an empty cage-like complex with the tip of the trimer directed toward the center of the structure.Examination of the 4-Å resolution crystal structures of dodecahedral truncated E 2 (tE 2 ) cores from Enterococcus faecalis and B. stearothermophilus and the 2-Å resolution crystal structure of a cubic tE 2 core from Azotobacter vinelandi...