Conformational Heterogeneity and DNA Recognition by the Morphogen Bicoid.

Area of Science:

• Developmental Biology
• Molecular Biology
• Protein Structure and Dynamics

Background:

• The bicoid (Bcd) protein is a crucial transcription factor in Drosophila development.
• Bcd's homeodomain mediates DNA binding but uniquely interacts with multiple DNA sequences and RNA.
• Previous studies suggested conformational heterogeneity in Bcd's DNA-binding residues underlies its polyspecificity.

Purpose of the Study:

• To directly characterize the conformational heterogeneity of the Bcd homeodomain.
• To investigate the role of conformational flexibility in Bcd's DNA and RNA binding.
• To understand the structural basis of Bcd's unique morphogenic activity.

Main Methods:

• Introduction of C-D bonds into each structural element of the Bcd homeodomain.
• Characterization of C-D bond absorptions in free, bound, and thermally denatured states.
• Analysis of conformational environments within the homeodomain's structural elements.

Main Results:

• Residues in the first two helices of the Bcd homeodomain are well-defined and unaffected by DNA binding.
• Residues in the recognition helix adopt multiple stable conformations, both in the presence and absence of DNA.
• Conformational heterogeneity arises from a population of stable conformations, not local unfolding.

Conclusions:

• The recognition helix of the Bcd homeodomain possesses inherent conformational flexibility.
• This flexibility allows Bcd to bind diverse nucleic acid targets, explaining its polyspecificity.
• Understanding Bcd's conformational dynamics is key to its developmental role.