LIM Domain Binding Proteins 1 and 2 Have Different Oligomeric States

Abstract

LIM domain binding (Ldb) proteins are important regulators of LIM homeodomain and LIM-only proteins that specify cell fate in many different tissues. An essential feature of these proteins is the ability to self-associate, but there have been no studies that characterise the nature of this self-association. We have used deletion mutagenesis with yeast two-hybrid analysis to define the minimal self-association domains of Ldb1 and Ldb2 as residues 14–200 and 21–197, respectively. We then used a range of different biophysical methods, including sedimentation equilibrium and small-angle X-ray scattering to show that Ldb114–200 forms a trimer and Ldb221–197 undergoes a monomer–tetramer–octamer equilibrium, where the association in each case is of moderate affinity (∼105 M−1). These modes of association represent a clear physical difference between these two proteins that otherwise appear to have very similar properties. The levels of association are more complex than previously assumed and emphasise roles of avidity and DNA looping in transcriptional regulation by Ldb1/LIM protein complexes. The abilities of Ldb1 and Ldb2 to form trimers and higher oligomers, respectively, should be considered in models of transcriptional regulation by Ldb1-containing complexes in a wide range of biological processes.