The SPR2 C-terminal domain has a conserved face with partitioned charge.

(A) The SPR2 C-terminal domain shown in surface representation, with conservation from Fig 1B mapped on the surface (green: 100% identity; yellow: 100% similarity). Orientation at top as shown in Fig 3A (image at left), orientation below after a 180° rotation about the y-axis. (B) Conserved, hydrophobic, surface exposed determinants of the α1 helix (W723, W726, and SeMet730) are shown in stick format. Conservation is colored as in Fig 1B, with final 2mF_o-DF_c electron density shown in blue, contoured at 1.0 σ. (C) View of conserved residues in the α1–α3 region, highlighting the surface exposed hydrophobic and basic nature of the region. The basic residues R719 and R734 of α1, and K756 and K760 of α3 are labeled in blue font. Backbone shown in cartoon format, colored as in Fig 3A, with residues shown in stick format, colored as in Fig 1B. (D) View of conserved residues in the α3–α7 region, highlighting the conserved, surface exposed residues along α5, residue P763 of the α3-α4 loop, as well as residue W839 of the α6-α7 loop, which is positioned in a pocket on the side of the domain. The W839 side chain interacts with and is stabilized by residues that include L794, P842, P844, and L847. Backbone shown in cartoon format, colored as in Fig 3A, with conserved residues shown in stick format, colored as in Fig 1B. (E) Electrostatic surface potential mapped on the SPR2 C-terminal domain structure, views oriented as in A.