Kinetic factors and form determination of the head of bacteriophage T4.

Abstract

The form of the bacteriophage T4 prehead is described by its icosahedral symmetry, its diameter, and its length. We show how each oftheseparameters is regulated during prehead formation and ascribe specific form-determining functions to the prehead proteins. The major protein of the head shell can assemble in several different forms. The structure produced in vivo depends on the rate of synthesis of the major protein relative to the rates of synthesis of minor shell proteins and the major core protein. From our observations, we propose a model for form determination of the prehead and suggest a pathway for the evolution of its prolate shape.The head of bacteriophage T4 has been extensively studied as a model for discovering mechanisms of form determination of biological structures. It is a prolate icosahedron of triangulation number (T) = 13 (1, 2) elongated on its 5-fold axis of symmetry (3), with a length-to-width ratio of 1.37 (4). It is assembled on the Escherichia colh inner membrane as a core-containing prehead (5), which is matured to the finished head by a series of reactions that includes limited proteolysis of capsid and core proteins, expansion of the prehead shell lattice, and packaging of the phage DNA (6). The relatively large size and the prolate shape of the T4 head suggest that the prehead [in which the form is already determined (4)] cannot be produced by simple self-assembly of the shell protein, as is possible with small regular icosahedral viruses (7,8).A number of mechanisms have been proposed (9, 10) to account for form determination in the assembly of large or anisometric virus shells or other structures in which simple selfassembly of identical subunits is not possible (11). They include regulation by a genetically defined "measuring rod" or template, a vernier, and accumulating strain with the addition of subunits. Of these, only the "measuring rod" model, (the determination of the length of tobacco mosaic virus by its RNA) has been experimentally demonstrated (12,13). All of these models depend strongly on specific interactions between the polymerizing subunits, but Wagenknecht and Bloomfield (14) have shown theoretically how the length of linear aggregates might also depend on the concentration of the polymerizing subunits.Recent observations on the role of minor proteins in prehead formation, and on changes in prehead length that depend on subunit concentrations, suggest that kinetic factors as well as specific protein-protein interactions regulate the form of the T4 prehead.The form of the T4 prehead is described by its symmetry (icosahedral), its width, and its length (extension along the 5-fold axis of symmetry). We account for these in the following way. The symmetry is determined by formation of a 5-fold symmetric initiation complex that directs assembly of the shell into closed structures instead of open tubes. The width is determined by the intrinsic curvature of the shell and the diameter of the core that it must enclose. The length can be varied. It is regulated by t...

Biocontrol of Listeria monocytogenes and Escherichia coli O157:H7 in Meat by Using Phages Immobilized on Modified Cellulose Membranes

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Involvement of the Portal at an Early Step in Herpes Simplex Virus Capsid Assembly

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Expression of virus-encoded proteinases: functional and structural similarities with cellular enzymes.

“…The prohead is loosely assembled with a precursor form of gene product gp21. gp2l is activated by autodigestion and is the proteinase responsible for cleavage of at least six other prohead proteins (318)(319)(320)(321). Concurrent with cleavage, DNA is packaged and the ultimate product, a stable packaged head structure, is formed.…”