The gpQ portal protein of bacteriophage P2 forms dodecameric connectors in crystals

Double-stranded bacteriophages code for a protein called a connector or portal protein that serves as the entry and exit portal for DNA during genome packaging and ejection, as well as the connection point between heads and tails, and possibly as a nucleator for capsid assembly. The gpQ connector protein from bacteriophage P2 has been overexpressed in Escherichia coli and purified by sucrose gradient centrifugation. Negative stain electron microscopy and image analysis revealed a 135 Å diameter dodecameric ring structure with a central 25 Å hole. The connector showed a strong propensity to aggregate at low ionic strength and would form microcrystalline structures in solution. Consequently, the connectors were crystallized by hanging-drop vapor diffusion against low ionic strength buffer. Two crystal forms were observed: a P4122 form with unit cell parameters a = b = 96.33 Å and c = 454.42 Å that diffracted X-rays to 4.5 Å resolution and an I222 crystal form with a = 168.86 Å, b = 171.88 Å and c = 168.68 Å that diffracted to 4.1 Å resolution. Self-rotation functions confirmed the presence of 12-fold symmetry in the crystals.