Three-dimensional structure of bacteriophage T4 baseplate

The baseplate of bacteriophage T4 is a multiprotein molecular machine that controls host cell recognition, attachment, tail sheath contraction and viral DNA ejection. We report here the three-dimensional structure of the baseplate tail tube complex determined to a resolution of 12 Angstrom by cryoelectron microscopy. The baseplate has a six-fold symmetric, dome-like structure -520 Angstrom in diameter and -270 Angstrom long, assembled around a central hub. A 940 Angstrom long and 96 Angstrom diameter tail tube, coaxial with the hub, is connected to the top of the baseplate. At the center of the dome is a needle-like structure that was previously identified as a cell puncturing device. We have identified the locations of six proteins with known atomic structures, and established the position and shape of several other baseplate proteins. The baseplate structure suggests a mechanism of baseplate triggering and structural transition during the initial stages of T4 infection.