Genetic control of bacteriophage T4 baseplate morphogenesis: III. Formation of the central plug and overall assembly pathway

Abstract

Cells infected with bacteriophage T4 amber mutants defective in any of seven baseplate genes (5, 25, 26, 51, 27, 28 and 29) are unable to form functional baseplates because of a defect in the assembly of the central part of the baseplate. Upon mixing concentrated extracts of such mutant-infected cells, complete baseplates, and subsequently, viable phage, assemble in vitro. By characterizing the in vitro assembly activity of the precursor complexes accumulating in the mutant-infected cells, we have been able to define the gene product interactions in morphogenesis. The pathway for the interaction of the six gene products which form the central part of the baseplate is as follows: the gene 29 protein is acted upon by the gene 26 and 28 products, yielding a 7 S precursor. The action of the gene 51 product converts this to a 14 S complex. Independently, the gene 27 and gene 5 products assemble into a 12 S complex. This 12 S 5+:27+ complex then combines with the 14 S 29+:28+:26+:51+ complex to form the final 22 S complex. This structure probably represents the hexameric central plug or hub of the baseplate. A number of the proteins probably act catalytically and are not present in the final complex. Three of the gene products gp29, gp5, and gp27† are incorporated into the complex. These results taken together with those of the previous papers show that the overall pathway for the morphogenesis of the T4 baseplate represents the aggregation of six 15 S arm complexes around a central 22 S hub structure. Two more gene products add to the outer face of the baseplate, and two other gene products to the inner face, only after an hexagonal structure has organized. Five of the six genes specifying hub proteins are contiguous on the phage chromosome. Similarly, six of the seven genes specifying “arm” proteins are contiguous on the chromosome. The odd gene in each set maps at one end of the opposite cluster. As a result of these experiments, we can now define the complete pathway for the interactions of 22 T4 gene products involved in the morphogenesis of the phage tail. The sequential nature of this pathway is striking; proteins only interact with each other if one member has already been incorporated into a growing structure. Thus, the regulation of assembly appears to be entirely at the level of protein-protein interactions, and the genetic control of this particular morphogenetic process operates at the level of primary protein sequence specification.