ABSTRACT
Kikuchi, Y and King, J. (1975) J Mol Biol., 99(4), 645-672
Genetic control of bacteriophage T4 baseplate morphogenesis. I. Sequential assembly of the major precursor, in vivo and in vitro
The proteins specified by at least 18 bacteriophage T4 genes are needed for the formatin of the complex hexagonal baseplate of the phage tail. At least 14 of these protein species are incorporated into the mature structure of this organelle.
Cells infected with amber mutants defective in any of five culstered baseplate genes (53, 6, 7, 8 and 10) are unable to form hexagonal structures. These mutant-infected cells accumulate structural protein complexes which are precursors to baseplates. they can be assembled into mature baseplates and thus viable phage, in vitro, in mixtures of extracts of mutant-infected cells.
We have isolated the precursor proteins and protein complexes specified by the five genes, by gradient centrifugation. The proteins have been characterized by sodium dodecyl sulfate gel electrophoresis, and by their in vitro assembly activity. The results show that the major structural precursor of the baseplate is a 15 S complex containing six protein species. These interact in the following sequence:
| gp53 |
(23,000 mol. wt)
|
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| gp6 | ||||||||||||
| gp8 | gp6 |
(85,000 mol. wt)
|
||||||||||
| gp7 | gp8 | |||||||||||
| gp10 | gp7 | gp8 |
(46,000 mo. wt)
|
|||||||||
| ---> | gp10 | ---> | ---> | gp10 | ---> | ---> | gp7 | ---> | ||||
| gp11 | gp10 | gp7 |
(140,000 mol. wt)
|
|||||||||
| gp11 | gp10 | |||||||||||
| gp11 | gp10 |
(88,000 mol. wt)
|
||||||||||
| gp11 | ||||||||||||
| gp11 |
(42,000 mol. wt)
|
With the exception of gp11, which is not an obligatory protein in the pathway, the proteins can interact only in this order. Protein composition and electron microscope morphology indicate that the 15 S complex is the structure which polymerizes radially to form the hexagonal 70 S baseplate, gp is gene product.
If any step in this pathway is blocked by mutation, the subsequent proteins remain soluble and unassembled. With the exception of the initiating proteins, reactive sites are not present on the soluble subunits, but are generated only upon incorporation of a subunit into the growing complex. Thus reactive sites are limited to growing structures, ensuring efficient assembly.
