Characterization of homo- and heterodimerization of cardiac Csx/Nkx2.5 homeoprotein

Csx/Nkx2.5 is an evolutionarily conserved homeodomain (HD)-containing trans cription factor that is essential for early cardiac development. We found t hat the HD of Csx/Nkx2.5 binds as a monomer as well as a dimer to its DNA b inding sites in the promoter of the atrial natriuretic factor (ANF) gene, a n in vivo target gene of Csx/Nkx2.5. Csx/Nkx2.5 physically interacts with e ach other in vitro as well as in cells, and the HD is critical for homodime rization. Lys(193) and Arg(194), located at the COOH-terminal end of HD, ar e essential for dimerization. Lys193 is also required for a specific intera ction with the zinc finger transcription factor GATA4. Csx/ Nkx2.5 can hete rodimerize with other NK2 homeodomain proteins, Nkx2.3 and Nkx2.6/Tix, with different affinities. A single missense mutation, Ile(183) to Pro in the H D of Csx/Nkx2.5, preserved homodimerization function, but totally abolished DNA binding. Ile(183) --> Pro mutant acts in an inhibitory manner on wild type Csx/ Nkx2.5 transcriptional activity through the ANF promoter in 10T1/ 2 cells. However, Ile(183) --> Pro mutant does not inhibit wild type Csx/Nk x2.5 function on the ANF promoter in cultured neonatal cardiac myocytes, po ssibly due to failure of dimerization in the presence of the target DNA. Th ese results suggest that complex protein-protein interactions of Csx/Nkx2.5 play a role in its transcriptional regulatory function.