Abstract
We have used translational diffusion coefficient measurements and subunit hydrodynamic theory to determine the dimensions and shape of bacterioophage T4D baseplates and tails. The diffusion coefficient of the baseplate, measured by quasielastic laser light scattering (QLS), was determined previously by Wagenknecht and Bloomfield to be D = 8.56 ร 10^โ8^ cm^2^/s. For the tail, we found D = 5.88 ร 10^โ8^ cm^2^/s by QLS, and D = 6.02 ร 10^โ8^ cm^2^/s by combining sedimentation coefficient and molecular weight in the Svedberg equation. These values, which have an uncertainty of ยฑ2.7%, when combined with subunit hydrodynamic theory, enabled us to refine estimates of dimensions obtained by electron microscopy. For the hexagonal baseplate, the vertexโtoโvertex distance is about 480 ร
, the thickness is 160 ร
, and there are six extended short fibers 320โร
long and 40 ร
in diameter. When a baseplate of these dimensions is attached to a tail tubeโsheathโconnector complex 1050โร
long and 240 ร
in diameter, the calculated D is 5.93 ร 10^โ8^ cm^2^/s, within 1% of experiment. This combined use of electron microscopy and hydrodynamics, using the former to ascertain shape, and the latter to obtain solution dimensions, is a powerful approach to the structure of biomolecular complexes.