Ultrastructure of the eukaryotic aminoacyl-tRNA synthetase complex derived from two dimensional averaging and classification of negatively stained electron microscopic images.

Several aminoacyl-tRNA synthetases in higher eukaryotes are consistently isolated as a multi-enzyme complex for which little structural information is yet known. This study uses computational methods for analysis of electron microscopic images of the particle. A data set of almost 2000 negatively stained images was processed through reference-free alignment and multivariate statistical analysis. Interpretable structural information was evident in five eigenvectors. Hierarchical ascendant classification extracted clusters corresponding to distinct image orientations. The class averages are consistent with rotations around and orthogonal to a central particle axis and provide particle measurements: approximately 25 nm in height, 30 nm at the widest point and 23 nm thick. The results also provide objective evidence in support of the working structural model and demonstrate the feasibility of obtaining the three dimensional structure of the multisynthetase complex by single particle reconstruction methods.