Med18 is one subunit of Mediator, a head-module multiprotein complex, that stimulates basal RNA polymerase II (Pol II) transcription. Med18 consists of an eight-stranded beta-barrel with a central pore and three flanking helices. It complexes with Med8 and Med20 proteins by forming a heterodimer of two-fold symmetry with Med20 and binding the C-terminal alpha-helix region of Med8 across the top of its barrel. This complex creates a multipartite TBP-binding site that can be modulated by transcriptional activators [1].

1: Structure and TBP binding of the Mediator head subcomplex Med8-Med18-Med20. Lariviere L, Geiger S, Hoeppner S, Rother S, Strasser K, Cramer P; Nat Struct Mol Biol. 2006;13:895-901. PMID:16964259