Pseudomonas phage phi KZ and its two close relatives phi PA3 and 201 phi 2-1 are very large bacteriophages that form a separate branch in phage classification because their genomes are very different from the rest of GenBank sequence data. The contractile tail of phi KZ is built from at least 32 different proteins, but a definitive structural function is assigned to only one of them-the tail sheath protein. Here, we report the crystal structure of the C-terminal domain of another phiKZ tail protein, gene product 131 (gp131C). We show that gp131 is located at the periphery of the baseplate and possibly associates with fibers that emanate from the baseplate. Gp131C is a seven-bladed beta-propeller that has a shape of a skewed toroid. A small but highly conserved and negatively charged patch on the surface of gp131C might be important for substrate binding or for interaction with a different tail protein. (C) 2012 Elsevier Inc. All rights reserved.