Structure and Assembly of Bacteriophage T4 Tail Fibers

Abstract

The structure and assembly of bacteriophage T4 tail fibers was examined as a model system for studying assembly of multiprotein structures. The function of six genes (34- 38 and 57) is necessary for assembly of tail fibers. The role of each of these genes in assembly was examined by isolating and characterizing whole fibers and four precursors which accumulate in lysates of mutant phage-infected bacteria. Each isolated structure showed a single major electrophoretic component on polyacrylamide gels indicating near homogeneity. Electron microscopic examination of these isolated structures revealed that the whole fiber consists of two halves; one, requiring genes 57 and 34 for its assembly, is a rod 690 x 27 A with a knob on one end and an antigen A; the other, requiring genes 35, 36, 37 and 57 for its assembly, is a rod 690 x 26 A containing antigens B and C (designated BC'). The antigen B is introduced into this rod by the action of gene 36 which increases the length of a half fiber precursor, C, the product of genes 37, 38 and 57, from 560 to 690 A. The resulting BC precursor is converted to BC' under the control of gene 35, a step necessary to allow interaction with the A half fiber, but one making no morphological or serological change in the fiber. The subunit composition of the isolated fibers and their precursors was examined by dissociation at l00°c and gel electrophoresis in the presence of the anionic detergent sodium dodecyl sulfate (SDS). A was found to contain a major polypeptide of molecular weight 150,000. C, BC and BC' were each found to contain a major polypeptide of molecular weight 123,000. Minor components were also present but they were not reproducible. Because the major polypeptides were so large they could be resolved on SDS gels of crude lysates of mutant infected cells. These gels showed that amber mutations in gene 34 eliminated the 150,000 polypeptide and amber mutations in gene 37 eliminated the 123,000 polypeptide. This indicates that these polypeptides are the products of genes 34 and 37 (P34 and P37). The other tail fiber genes did not affect the synthesis of P34 and P37 except that amber mutations in gene 36 reduced the amount of P37 by 40%, suggesting that these genes are co-transcribed. Molecular weight estimates of the fiber precursors show that there are two copies of P34 in the A half fiber and two copies of P37 in each of the other half fibers. Mutations in genes 57 and 38 affected the apparent solubility of P34 and P37 and allow these polypeptides to be dissociated in SDS at 37°C. This is consistent with P57 controlling the dimerization of P34 leading to the A half fiber and P38 and P57 controlling the dimerization of P37 leading to the C half fiber. A new apparatus for destaining acrylamide gels electrophoretically and a new method of fractionation and scintillation counting of radioisotope-labeled acrylamide gels are also described.