Insights into dimerization and four-helix bundle formation found by dissection of the dimer interface of the GrpE protein from Escherichia coli

The GrpE heat shock protein from Escherichia coli has a homodimeric structure. The dimer interface encompasses two long α-helices at the NH₂-terminal end from each monomer (forming a "tail"), which lead into a small four-helix bundle from which each monomer contributes two short sequential α-helices in an antiparallel topological arrangement. We have created a number of different deletion mutants of GrpE that have portions of the dimer interface to investigate requirements for dimerization and to study four-helix bundle formation. Using chemical crosslinking and analytical ultracentrifugation techniques to probe for multimeric states, we find that a mutant containing only the long α-helical tail portion (GrpE1-88) is unable to form a dimer, most likely due to a decrease in α-helical content as determined by circular dichroism spectroscopy, thus one reason for a dimeric structure for the GrpE protein is to support the tail region. Mutants containing both of the short α-helices (GrpE1-138 and GrpE88-197) are able to form a dimer and presumably the four-helix bundle at the dimer interface. These two mutants have equilibrium constants for the monomer - dimer equilibrium that are very similar to the full-length protein suggesting that the tail region does not contribute significantly to the stability of the dimer. Interestingly, one mutant that contains just one of the short α-helices (GrpE1-112) exists as a tetrameric species, which presumably is forming a four-helix bundle structure. A proposed model is discussed for this mutant and its relevance for factors influencing four-helix bundle formation.