Form determination of the heads of bacteriophages

Abstract

The shape of the DNA-containing heads of many bacteriophages is not only determined by the properties of the protein subunits which build the shell (capsid) but also by the scaffolding core which is a transient structure of the prohead. The form-determining properties of the scaffolding proteins have been characterized by genetic methods based on conditional mutants and site-directed mutagenesis. The mechanism of form determination has been studied by in vitro assembly experiments. The theoretical background is discussed and different models for mechanisms of form determination are considered. Definitive decisions about the validity of a model is still limited by the difficulty of obtaining unambiguous answers on the stoichiometry and the fine structure of the scaffold because of their high instability.The conservation of the specific shape of any living species is the very basic observation that gave rise to the notion of genetics. The mechanisms which underlay form determination still remain among the fundamental unsolved problems of biology although the descriptive part, the genesis of shape (morphogenesis) from the fertilized egg to the fully grown organism, is known in detail for most multicellular organisms. In recent decades one has also learned that the different steps of the morphogenesis of multicellular organisms must be determined through the successive switching on and off of groups of genes. In consequence, toddy's research efforts are concentrated on the mechanisms of the genetic control of gene expression. Only once these are understood and the newly expressed genes identified, can the functions of the corresponding gene products, the proteins, be studied. The phenomena of cellular recognition and of contact inhibition of growth must be understood before one is even able to formulate plausible hypotheses for the form determination of multicellular organs and organisms. How, for example, does a regenerating, partly ectomized liver, know how to stop cellular proliferation when the correct shape is again achieved?The situation is much better, although quite different, for multi-molecular structures represented by elements of the cytoskeleton or by other cellular organelles and, above all, by viruses [l -31.Everybody is more or less familiar with what is called selfassembly of a multimolecular structure. It is represented, for instance, by some cases of virus capsids, where the shape and binding properties of the protein subunit are such that they can assemble only into one type of hollow sphere. Such cases are considered as form determination of the first order. This situation is somewhat trivial; indeed, for the large majority of viruses, form determination is not so simple: inner scaffolds are transiently needed to help in imposing any form to the capsid. The form of the capsid is therefore determined by the information contributed by both the properties of the subunitsCorrespondence to E. Kellcnberger, Department of Microbiology, Biozentrum, University of Basel, Klingelbergstrasse 70, CH-4056 Bas...

Role of electrostatic interactions in the assembly of empty spherical viral capsids

“…In principle, viral proteins can assemble in a variety of capsids with different shape and size 5,8,9 . We shall concentrate on the nearly spherical viral capsids whose structure can be described within the so-called Caspar-Klug quasiequivalent construction 7 .…”

A P22 Scaffold Protein Mutation Increases the Robustness of Head Assembly in the Presence of Excess Portal Protein

“…By starting the assembly of the procapsid at the portal vertex, each structure would acquire only one portal. This "nucleation" model was supported by experiments with the prolate bacteriophage T4, in which it was determined that portal protein was required for the formation of prolate heads (17,31). Additional support for a portal initiation model came from observations that the phage lambda portal protein promoted head assembly in cell lysates (20,24,31).…”

“…For example, during the assembly of the prolate phage 29, the absence of portal protein leads to the formation of isometric particles and spiral "monsters" (6,21). This dependence on portal protein has also been observed for phages T3, , and SPP1 (11,17,31). In the absence of the phage T4 portal protein, long, open-ended tubes of coat protein form that contain a scaffold core (20).…”

“…In most other instances, when portal proteins are absent during assembly, coat and scaffold proteins assemble into aberrant spiral structures or long open-ended tubes (17,31). Therefore, portal proteins appear to play some role in head form determination.…”

ϕX174 Procapsid Assembly: Effects of an Inhibitory External Scaffolding Protein and Resistant Coat Proteins In Vitro

“…For example, coat proteins from other systems form aberrant assemblies in the absence of scaffolding protein, whereas scaffolding proteins alone are relatively inert. When mixed, the scaffolding proteins control fidelity, suppressing the formation of heterogeneous aberrant structures containing only the coat protein (21,33,(42)(43)(44)(45)(46)(47)(48)(49)(50). In contrast, the X174 external scaffolding protein D self-associates to form large heterogeneous spherical complexes (Fig.…”