Capsid proteins cleavage from polyprotein

If the entire picornavirus genome is translated into a single giant polyprotein which is cleaved to form the easily detectable primary proteins, A, F and C, then these proteins and their cleavage products should appear in a molar ratio of 1 1 1. If one calculates the molar ratios of the primary products as mentioned earlier, the results shown in Table 2 are obtained for mengovirus f22, 25), EMC virus (25) human rhinovirus 1A (25) and poliovirus (59) The A F C ratio for EMC virus (25) was found to be close to the theoretical value of 1 1 1. That for HRV-1A was also close to the theoretical value, 0.85 1.00 0.44> though the value for C was low. In contrast, the ratios for mengovirus (determined in two independent laboratories) and poliovirus deviated from the expected values. The capsid proteins appeared to be overproduced nearly by a factor of 2. 

Herpesviruses encode a serine protease that is essential for the maturation of viral capsids (1,2). The protease is expressed as part of a polyprotein. The catalytic domain is contained within the N-terminal third of the protein, and the remainder comprises a structural scaffold protein. The scaffold protein is independently expressed in excess to the polyprotein from an internal initiation codon. The protease cleaves the polyprotein at two sites one at the c-terminus of the protease catalytic domain, the release or R-site, and the other close to the c-terminus of the scaffold protein, the maturation or M-site (Fig. 1). Cleavage of the M-site follows assembly of the viral procapsids and precedes packaging of the viral DNA. The M-site sequence is conserved among the herpesviruses and has a consensus sequence (V/L)-X-A-S, with cleavage between A-S (3). Structural studies have shown that the herpesvirus proteases have a novel structure, and their essential role in capsid maturation makes them a potential target for antiviral intervention. 

---