Understanding the polymorphic behaviour of a mutant of the α-spectrin SH3 domain by means of two 1.1 Å resolution structures

SH3 domains are small protein modules that mediate the assembly of specific protein complexes, typically via binding to proline-rich sequences in their respective binding partners. Most of the α-spectrin SH3-domain (Spc-SH3) structures determined to date using X-ray diffraction have been solved from crystals belonging to the orthorhombic space group P2₁2₁2₁ with a needle-like morphology. All of these ortho­rhombic crystals exhibited a rapid growth rate. In addition to this crystal form, the R21D mutant of Spc-SH3 crystallizes in a new crystal form in the presence of sodium formate at pH values higher than 6. This new crystal form grows at a slower rate and belongs to the hexagonal space group P6₅22, with unit-cell parameters a = b = 42.9, c = 127.5 Å. When both polymorphs of the R21D mutant of Spc-SH3 are simultaneously present into the same solution, it has been observed that the hexagonal crystals grow at the expense of the orthorhombic crystals. The availability of 1.1 Å resolution structures for both crystal forms allows the identification of key features that could account for the observed polymorphic behaviour.