X-Ray crystallographic structure and deletion mutagenesis of LacI have demonstrated that three-leucine-heptad repeats (LHR3), located at the C-terminus, are critical for dimer-dimer interaction to form tetramer. We are interested in exploring the potential for dimeric proteins, such as purine repressor, which share significant homology to lac repressor, to form higher oligomerization states when this assembly element is present. This information will expand our understanding of the general mechanism of protein oligomer formation.
To assess whether introduction of a LHR motif will elicit tetramer formation, two mutants of PurR, PL3 and PL3+, with different lengths of the LHR3 sequences of lac repressor, have been constructed. A monomer-dimer-tetramer equilibrium is observed for PL3, along with wild-type corepressor binding affinity and decreased operator binding affinity. PL3+ is a trimer, and has no detectable corepressor binding affinity and decreased operator binding affinity. These data indicate that construction of higher order oligomers may be complex.