Self-association of S100A4Δ8 molecules in the crystal lattice. (A) The Ca2+-S100A4Δ8 dimer is shown with one red and one gray subunit. The S100A4Δ8C dimers in the crystal are positioned such that the C-terminal tail of subunit B (red) inserts into the hydrophobic cleft (subunit A-gray) of the adjacent S100A4 dimer. Similarly, the C-terminal tail of subunit A (gray) from the same adjacent dimer inserts into the hydrophobic cleft on subunit B (red). (B) High magnification view of the boxed area in (A). Phe89 and Phe90 of the C-terminal tail are buried inside the hydrophobic cleft of the symmetry-related molecule. (C) Superimposition of the S100A4/MIIA1893-1935 structure (3ZWH; peptide – green, S100A4 dimer – orange and pistachio) onto the S100A4Δ8C/MIIA1908-1923 structure (peptide – blue, S100A4Δ8C dimer – gray and red). (D) Myosin-IIA residues Pro1927-Val1930 (green) from the S100A4/MIIA1893-1935 structure (3ZWH) occupy the same region of the hydrophobic cleft as the C-terminal tail of S100A4Δ8C. S100A4Δ8C residues Glu88-Glu91 (magenta) and myosin-IIA residues Pro1927-Val1930 (green) adopt a similar conformation. Note that the S100A4 (red) and myosin-IIA (green) helices approach the hydrophobic cleft from opposite orientations.