Abstract
Iroquois Homeobox 4 (IRX4) belongs to a family of homeobox TFs having
roles in embryogenesis, cell specification and organ development.
Recently, Large scale Genome-Wide Association studies and epigenetic
studies have highlighted the role of IRX4 and its associated
variants in prostate cancer. No studies have investigated and
characterized the structural aspect of the IRX4 homeodomain and its
potential to bind to DNA. The current study uses sequence analysis,
homology modelling and molecular dynamics simulations to explore IRX4
homeodomain-DNA recognition mechanisms and the role of somatic mutations
affecting these interactions. Using publicly available databases, gene
expression of IRX4 was found in different tissues, including
prostate, heart, skin, vagina, and the protein expression was found in
cancer cell lines (HCT166, HEK293), B cells, ascitic fluid and brain.
Sequence conservation of the homeodomain shed light on the importance of
N- and C-terminal residues involved in DNA binding. The specificity of
IRX4 homodimer bound to consensus human DNA sequence was confirmed by
molecular dynamics simulations, representing the role of conserved amino
acids including R145, A194, N195, S190, R198 and R199 in binding to DNA.
Additional N-terminal residues like T144 and G143 were also found to
have specific interactions highlighting the importance of N-terminus of
the homeodomain in DNA recognition. Additionally, the effects of somatic
mutations, including the conserved Arginine (R145, R198 and R199)
residues on DNA binding elucidated the importance of these residues in
stabilizing the protein-DNA complex. Secondary structure and hydrogen
bonding analysis showed the roles of specific residues (R145, T191,
A194, N195, R198 and R199) in maintaining the homogeneity of the
structure and its interaction with DNA. The differences in relative
binding free energies of all the mutants shed light on the structural
modularity of this protein and the dynamics behind protein-DNA
interaction. We also have predicted that the C-terminal sequence of the
IRX4 homeodomain could act as a potential cell-penetrating peptide,
emphasizing the role these small peptides could play in targeting
homeobox TFs.