Three‐dimensional Domain Swapping in the α Subunit of Tryptophan Synthase

Abstract

Tryptophan synthase is an enzyme participating in the last two steps of tryptophan synthesis in plants, fungi and bacteria. It consists of two protein chains, α and β, encoded by trpA and trpB genes, that function as an αββα complex. Structural and functional features of tryptophan synthase have been extensively studied, explaining roles of individual residues in catalysis and allosteric regulation of the two active sites. TrpA has become a model system to investigate folding mechanisms of larger proteins. In 1969, Jackson and Yanofsky observed that typically monomeric TrpA forms a small population of dimers. The dimerization was postulated to occur through an exchange of structural elements of the monomeric chains – a phenomenon currently known as 3D domain swapping. Nevertheless, structural details of the dimeric form of TrpA protein remained unknown. We determined the crystal structure of TrpA that for the first time illustrates domain swapping in a TIM barrel fold. The exchanging fragment represents the N‐terminal section and includes H0‐S1‐H1‐S2 elements. The hinge region corresponds to loop L2 linking S2 with H2'. S2 and L2 carry the catalytic residues, Glu and Asp. As the S2 element is part of a swapped domain, architecture of the catalytic apparatus in α2 is regenerated from the two protein chains. The dimer interface corresponds to the α‐β interface in the tryptophan synthase complex, suggesting that the observed α2 dimer may not form a functional complex with the β subunit.This work was supported by the NIH grant GM094585 (MCSG) and the U.S. DOE, OBER, contract DE‐AC02‐06CH11357 (SBC).