Crystal structure of an isoleucine-zipper trimer

Area of Science:

• Structural biology
• Protein biochemistry
• Molecular genetics

Background:

• Subunit oligomerization is crucial for protein function and is often mediated by coiled-coil motifs.
• Coiled-coil motifs exhibit a seven-amino-acid repeat with hydrophobic residues at 'a' and 'd' positions.
• Sequence variations at these positions lead to diverse oligomerization states, including dimers, trimers, and tetramers.

Purpose of the Study:

• To investigate the molecular basis determining the oligomerization state (dimer, trimer, tetramer) of coiled-coil proteins.
• To characterize GCN4 leucine-zipper variants with mutations at 'a' and 'd' positions to understand oligomer choice.
• To elucidate the structural features that favor trimeric coiled-coil formation.

Main Methods:

• Characterization of GCN4 leucine-zipper variants with specific mutations at the 'a' and 'd' positions.
• High-resolution X-ray crystallography to determine the three-dimensional structure of mutant proteins.
• Analysis of amino acid residue packing within the hydrophobic core of different coiled-coil conformations.

Main Results:

• A specific isoleucine-containing mutant was found to form a parallel three-stranded alpha-helical coiled coil.
• The interior packing of the trimeric coiled coil accommodates beta-branched residues at hydrophobic positions.
• This accommodation contrasts with the packing constraints observed in dimer and tetramer structures.

Conclusions:

• The oligomerization state of GCN4 leucine-zipper variants is determined by the compatibility of core amino acid shapes with specific packing spaces.
• Beta-branched residues are favorably accommodated in the distinct core environment of a trimeric coiled coil.
• Understanding residue-packing compatibility provides insight into the sequence-structure relationships governing protein quaternary structure.