Replication complex association with membranes

Viral ERA synthesis in the picornavirus group is associated with membrane structirres (25, 26) and can be isolated in the form of a replication complex by centrifugation of the cytoplasmic extracts at 20,000 g (27). The phosphorylase activity was shown to be associated with the replication complexes (Table 7)> result expected, as dsRRA structures were in the P-20 fraction. 

The complete complex of nucleic acid and protein, packaged in the virus particle, is called the virus nucleocapsid. Although the virus structure just described is frequently the total structure of a virus particle, a number of animal viruses (and a few bacterial viruses) have more complex structures. These viruses are enveloped viruses, in which the nucleocapsid is enclosed in a membrane. Virus membranes are generally lipid bilayer membranes, but associated with these membranes are often virus-specific proteins. Inside the virion are often one or more virus-specific enzymes. Such enzymes usually play roles during the infection and replication process. 

If idENA has the same nucleotide sequence as virion MA, why-are mRNA strands not encapsidated into virions A trivial explanation would be that viral MA, once it has combined with ribosomes, may subsequently not exist free of them. However, free viral MA (EHA not bound to ribosomes) that is 5 "terminated with pUp has been found in cytoplasmic extracts of infected cells (21). Thus there exists the intriguing possibility that, in addition to the role in replication, VPg linked to the 5 end of plus-strand EHA is also a prerequisite for virion formation. For example, the newly synthesized Pg-MA may associate with one or several capsid protomers (42) prior to encapsidation. This protomer/VPg-EHA complex could then protect the YPg(Tyr)-0 pU linkage from cleavage by the host cell enzyme. Consequently, the concentration of protomers would determine how much of the newly synthesized VPg-RNA is "processed" to mENA, that is, early in infection most if not all progeny EHA may be cleaved at the 5 end. If VPg linked to EHA is a prerequisite for encapsidation, why do virions not contain minus strands A simple explanation could be provided by the observation that newly synthesized minus strands are rapidly incorporated into El and RP structures inside the membrane-bound replication complex minus strands are not found as free ENA in the cytoplasm (5) and are thus neither cleaved at the 5 end nor encapsidated. 

As an oligo(A)-primed, terminal adenylate transferase of cellular origin was found in tight association with the same membranes the replication complex is bound to, the longer poly(A) stretches were thought to result from either a mechanism of "slippage" or by end addition of adenylate residues (89). 

All viral structures involved in picornavirus RNA synthesis are tightly associated with the smooth cytoplasmic membranes (50 Synthesis of viral RNA occurs exclusively in the replication complex (RC), a complex structure including the RNA template and a virus-coded RNA polymerase (52). 

Initial translation of the genomic RNA produces the nonstruc-tural polypeptides for viral RNA replication at the rough endoplasmic reticulum (RER) membrane. RNA replication also occurs in association with the membranes of vacuoles which develop in the infected cells (Grimley et al., 1968). These cytopathic vacuoles appear to be formed by fusion of various preexisting vacuoles. The viral RNA replication complex probably contains nonstructural proteins which have replicase and transcriptase activities including RNA chain elon-... 

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