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 angstrom diameter dodecameric ring structure with a central 25 A 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 P4(1)22 form with unit cell parameters a = b = 96.33 angstrom and c = 454.42 angstrom that diffracted X-rays to 4.5 angstrom resolution and an I222 crystal form with a = 168.86 angstrom, b = 171.88 angstrom and c = 168.68 angstrom that diffracted to 4.1 angstrom resolution. Self-rotation functions confirmed the presence of 12-fold symmetry in the crystals. (c) 2006 Elsevier Inc. All rights reserved.