The unusual fold of herpes simplex virus 1 UL21, a multifunctional tegument protein.

UL21 is a conserved protein in the tegument of alphaherpesviruses and has multiple important albeit poorly understood functions in viral replication and pathogenesis. To provide a roadmap for exploration of the multiple roles of UL21, we determined the crystal structure of its conserved N-terminal domain from herpes simplex virus 1 to 2.0-angstrom resolution, which revealed a novel sail-like protein fold. Evolutionarily conserved surface patches highlight residues of potential importance for future targeting by mutagenesis.