The FHA domain mediates phosphoprotein interactions.

The forkhead-associated (FHA) domain is a phosphopeptide-binding domain first identified in a group of forkhead transcription factors but is present in a wide variety of proteins from both prokaryotes and eukaryotes. In yeast and human, many proteins containing an FHA domain are found in the nucleus and involved in DNA repair, cell cycle arrest, or pre-mRNA processing. In plants, the FHA domain is part of a protein that is localized to the plasma membrane and participates in the regulation of receptor-like protein kinase signaling pathways. Recent studies show that a functional FHA domain consists of 120-140 amino acid residues, which is significantly larger than the sequence motif first described. Although FHA domains do not exhibit extensive sequence similarity, they share similar secondary and tertiary structures, featuring a sandwich of two anti-parallel (beta)-sheets. One intriguing finding is that FHA domains may bind phosphothreonine, phosphoserine and sometimes phosphotyrosine, distinguishing them from other well-studied phosphoprotein-binding domains. The diversity of proteins containing FHA domains and potential differences in binding specificities suggest the FHA domain is involved in coordinating diverse cellular processes.