Cloning, overexpression, purification and crystallization of the CRN12 coiled-coil domain from Leishmania donovani

The coiled-coil domain of glycosomal membrane-associated Leishmania donovani PEX14: cloning, overexpression, purification and preliminary crystallographic analysis

The glycosomal membrane-associated Leishmania donovani protein PEX14, which plays a crucial role in protein import from the cytosol to the glycosomal matrix, consists of three domains: an N-terminal domain where the signalling molecule binds, a transmembrane domain and an 84-residue coiled-coil domain (CC) that is responsible for oligomerization. CCs are versatile domains that participate in a variety of functions including supramolecular assembly, cellular signalling and transport. Recombinant PEX14 CC was cloned, overexpressed, affinity-purified with in-column thrombin cleavage and further purified by size-exclusion chromatography. Crystals that diffracted to 1.98 Å resolution were obtained from a condition consisting of 1.4 M sodium citrate tribasic dihydrate, 0.1 M HEPES buffer pH 7.5. The crystals belonged to the monoclinic space group C2, with unit-cell parameters a = 143.98, b = 32.62, c = 95.62 Å, β = 94.68°. Structure determination and characterization are in progress.

Structure of Leishmania donovani coronin coiled coil domain reveals an antiparallel 4 helix bundle with inherent asymmetry

Coiled coils are ubiquitous structural motifs that serve as a platform for protein-protein interactions and play a central role in myriad physiological processes. Though the formation of a coiled coil requires only the presence of suitably spaced hydrophobic residues, sequence specificities have also been associated with specific oligomeric states. RhXXhE is one such sequence motif, associated with parallel trimers, found in coronins and other proteins. Coronin, present in all eukaryotes, is an actin-associated protein involved in regulating actin turnover. Most eukaryotic coronins possess the RhXXhE trimerization motif. However, a unique feature of parasitic kinetoplastid coronin is that the positions of R and E are swapped within their coiled coil domain, but were still expected to form trimers. To understand the role of swapped motif in oligomeric specificity, we determined the X-ray crystal structure of Leishmania donovani coronin coiled coil domain (LdCoroCC) at 2.2Å, which surprisingly, reveals an anti-parallel tetramer assembly. Small angle X-ray scattering studies and chemical crosslinking confirm the tetramer in solution and is consistent with the oligomerization observed in the full length protein. Structural analyses reveal that LdCoroCC possesses an inherent asymmetry, in that one of the helices of the bundle is axially shifted with respect to the other three. The analysis also identifies steric reasons that cause this asymmetry. The bundle adapts an extended a-d-e core packing, the e residue being polar (with an exception) which results in a thermostable bundle with polar and apolar interfaces, unlike the existing a-d-e core antiparallel homotetramers with apolar core. Functional implications of the anti-parallel association in kinetoplastids are discussed.