Structural proteins of bacteriophage T4

The protein composition of purified T4 phage, phage heads and capsids was examined by sodium dodeoyl sulfate-acrylamide gel electrophoresis. The virion contains at least 21 unique proteins which range in molecular weight from 10,000 to 150,000. Both heads and capsids contain eight of these proteins. However, two of the eight proteins are partially released from heads by freeze-thaw or osmotic shock treatment. Thus, they are probably internal proteins. Capsids purified from cells infected with mutants defective in tail assembly can be separated into three fractions by cesium chloride equilibrium centrifugation. About 10% of the capsids have an unusually high density of from 1.35 to 1.40 g/cm³. Some of these capsids contain a collar. Below the collar are several 20 by 500 Å fibers (whiskers). The two other capsid fractions have densities of about 1.30 and 1.33 g/cm³. The capsids in these two fractions lack collars and whiskers. As a result of head assembly the major head structural protein, the gene 23 product, is converted from 55,000 to 46,000 molecular weight. The gene 23 product is not converted in cells infected with any of the head-defective amber mutants in genes 20, 21, 22, 24 and 31.