3D structure and significance of the G Phi XXG helix packing motif in tetramers of the E1 beta subunit of pyruvate dehydrogenase from the archeon Pyrobaculum aerophilum

As part Of a Structural genomics project, we have determined the 2.0 Angstr om structure of the E1 beta subunit of pyruvate dehydrogenase from Pyrobacu lum aerophilum (PA), a thermophilic archaeon. The overall fold of E1 beta f rom PA is closely similar to the previously determined E1 beta structures f rom humans (HU) and P. putida (PP). However, unlike the HU and PP structure s, the PA structure was determined in the absence of its partner subunit, E 1 alpha. Significant structural rearrangements occur in E1 beta when its E1 alpha partner is absent, including rearrangement of several secondary Stru cture elements such as helix C. Helix C is buried by E1 alpha in the HU and PP structures, but makes crystal contacts in the PA structure that lead to an apparent beta (4) tetramer. Static light scattering and sedimentation v elocity data are consistent with the formation of PA E1 beta tetramers in s olution. The interaction or helix C with its symmetry-related counterpart s tabilizes the tetrameric interface, where two glycine residues on the same race of one helix create a packing surface for the other helix. This G Phi XXG helix-helix interaction motif has previously been found in interacting transmembrane helices, and is round here at the E1 alpha -E1 beta interface for both the HU and PP alpha (2)beta (2) tetramers. As a case study in str uctural genomics, this work illustrates that comparative analysis of protei n structures can identify the structural significance of a sequence motif.