The apo-acyl coenzyme A binding protein of Leishmania major forms a unique 'AXXA' motif mediated dimer

Acyl-Coenzyme A binding domain containing proteins (ACBDs) are ubiquitous in nearly all eukaryotes. They can exist as a free protein, or a domain of a large, multidomain, multifunctional protein. Besides modularity, ACBDs also display multiplicity. The same organism may have multiple ACBDs, differing in sequence and organization. By virtue of this diversity, ACBDs perform functions ranging from transport, synthesis, trafficking, signal transduction, transcription, and gene regulation. In plants and some microorganisms, these ACBDs are designated ACBPs (acyl-CoA binding proteins). The simplest ACBD/ACBP is a small, ∼10 kDa, soluble protein, comprising the acyl-CoA binding (ACB) domain. Most of these small ACBDs exist as monomers, while a few show a tendency to oligomerize. In sync with those studies, we report the crystal structure of two ACBDs from Leishmania major, named ACBP_103, and ACBP₉₆ based on the number of residues present. Interestingly, ACBP₁₀₃ crystallized as a monomer and a dimer under different crystallization conditions. Careful examination of the dimer disclosed an exposed 'AXXA' motif in the helix I of the two ACBP₁₀₃ monomers, aligned in a head-to-tail arrangement in the dimer. Glutaraldehyde cross-linking studies confirm that apo-ACBP₁₀₃ can self-associate in solution. Isothermal titration calorimetry studies further show that ACBP₁₀₃ can bind ligands ranging from C₈ - to C₂₀-CoA, and the data could be best fit to a 'two sets of sites'/sequential binding site model. Taken together, our studies show that Leishmania major ACBP₁₀₃ can self-associate in the apo-form through a unique dimerization motif, an interaction that may play an important role in its function.