Biological Assembly, Ribosomes and Lambda Phage
We have already dealt with biological assembly in the folding of polypeptide chains. In that case, although we do not understand the process well enough to predict how a sequence of amino acids will fold, we have some of the basic principles well in hand. For most proteins, folding in the environment of the cell or in physiological buffers appears to be completely specified by the polypeptide itself. No external folding engine or scaffolding is required although some of the heat shock proteins assist folding by binding to proteins in unfolded states and preventing their irreversible aggregation. Is it also true that large bio-logical structures themselves contain all the information for their for-mation, or are scaffoldings and blueprints somehow needed?
Two fundamentally different types of larger structures are considered in this chapter: the structure and assembly of ribosomes and the structure and assembly of lambda phage. As we saw, ribosomes consist of two nonidentical subunits, each containing one or two RNA molecules and many different ribosomal proteins. The great majority of these proteins are present in each ribosome as a single copy. Thus, ribosomes are irregular and asymmetric. On the other hand, lambda phage, and most other viruses, possess a highly regular coat that covers DNA or RNA molecules. The coat of lambda consists of many copies of a few proteins that form an icosahedral head in addition to a number of other proteins that form a tail and tail fibers for absorption to cells. Both in the case of ribosome assembly and in the case of virus assembly, one important fundamental problem is determining the as-sembly order.