Polyprotein

Arnold, E., et al. Implications of the picornavirus capsid structure for polyprotein processing. Proc. Natl. Acad. ScL USA. 84 21-25, 1987. 

FIGURE 16.28 HIV mRNA provides the genetic information for synthesis of a polyprotein. Proteolytic cleavage of this polyprotein by HIV protease produces the individnal proteins required for viral growth and cellular infection. 

The infectious cycle of a (+)-strand RNA virus such as the hepatitis C virus differs by the fate of the viral RNA genome in the infected cell. Upon entry into the cell, the HCV genome is used as a messenger RNA to drive the synthesis of a large polyprotein precursor of about 3,000 residues [2]. The structural proteins are excised from the precursor by host cell signal peptidase. 

Rhinovirus, like poliovirus, synthesizes a large precursor protein from which all of the mature viral proteins are generated. Two viral proteases are involved in these cleavages 2A protease cleaves the polyprotein precursor at its own N terminus, while the 3C protease is responsible for additional cleavage events to generate the mature viral proteins. Both proteases can release themselves from the polyprotein precursor. Cleavage by 3C occurs between Gln/Gly, but flanking sequences affect efficiency (reviewed in Racaniello 2001). 

Matthews DA, Smith WW, Ferre RA, Condon B, Budahazi G, Sisson W, Villafranca JE, Janson CA, McElroy HE, Gribskov CL et al (1994) Structure of human rhinovirus 3C protease reveals a trypsin-like polypeptide fold, RNA-binding site, and means for cleaving precursor polyprotein. CeU 77 761-771... 

Yao N, Reichert P, Taremi SS, Prosise WW, Weber PC (1999) Molecular views of viral polyprotein processing revealed by the crystal structure of the hepatitis C virus bifunctional protease-heUcase. Structure 7 1353-1363... 

The HIV capsid is made up of auto-assembled protease-cleaved Gag polyprotein. This self-assembly cannot take place when appropriately positioned mutations are present, resulting in a drastically reduced infectivity of the progeny virus. Recently,... 

Douglass J, Civelli O, Herbert E Polyprotein gene expression Generation of diversity of neuroendocrine peptides. Annu Rev Biochem 1984 53 S65. 

The viral polyprotein comprises four structural proteins followed by six non-structural proteins. The structural proteins are envelope glycoproteins El and E2, Core, and the small P7 ion channel [14, 15]. Another putative protein, F, is also encoded by an alternate reading frame in the structural protein region, but no function for the protein has been identified [16]. The non-structural proteins are designated NS2-N5B. NS2/3 possesses an essential autoprotease activity, but it is still not clear whether NS2 itself has an independent function. The bifunctional NS3 protein consists of an N-terminal... 

Fig. 2.1 (A) Structure of the HCV genome, with 5 - and 3 -untranslated regions and individual proteins of the polyprotein indicated Regions of the genome (approximately 3,000 bases) are drawn to scale. Structural protein regions are in light gray, nonstructural proteins in white, and untranslated regions in dark gray. (B) Structure of a typical replicon sequence, with the antibiotic resistance gene (Neo ) in place of the structural region and the second IRES (EMCV) inserted. The NS2 sequence is often rwt present in the replicon.

A recent report has demonstrated that the proteolytic activity of NS3 plays an additional role in viral infection, beyond polyprotein processing. An important mediator of the cellular immune response is the transcription factor interferon regulatory factor 3 (IRF-3), which becomes activated on infection and then stimulates production of type-1 interferon and other antiviral genes [46]. It was found that expression of heterodimeric NS3/4A... 

HCV is a single-stranded RNA virus of the Flaviviridae family. It has a 9.4-kb positive-sense genome encoding a polyprotein precursor of 3011 amino acids. Individual isolates of HCV consist of closely related yet heterogeneous populations... 

Rhinoviri are the causal agents of common colds in humans. Viral replication and maturation is dependent on proteolytic processing of a viral polyprotein by a cysteine protease known as 3C protease. The active-site cysteine in 3C protease... 

Jasmer, D.P., Perryman, L.P. and McGuire, T.C. (1996) Haemonchus contortus GA1 antigens related phospholipase C-sensitive, apical gut membrane proteins encoded as a polyprotein and released from the nematode during infection. Proceedings of the National Academy of Sciences USA 93, 8642—8647. 

The remaining major classes of water-soluble lipid transporter proteins (other than the polyproteins of nematodes see below) come from plants and helminths. Plants possess very small (approximately 9 kDa) helix-rich, fatty-acid-binding proteins, the structures of some of which are known (Lerche and Poulsen, 1998). A recently described class comes from cestodes these are also very small (approximately 8 kDa), presumably intracellular, and helix-rich, and bind anthelmintic drugs in addition to fatty acids (Janssen and Barrett, 1995 Barrett et al., 1997). The only helix-rich small (approximately 14 kDa) lipid transporter from vertebrates is the acetyl-CoA-binding protein (Kragelund et al., 1993). 

The term polyproteins is used for two different types of entity. The first refers to precursor polypeptides which are cleaved post-translationally into biologically active proteins or peptides of quite different functions. Examples of these include polyproteins of viruses and some prohormones of vertebrates (reviewed in Kennedy, 2000b). The other type is large proproteins which comprise tandem repetitions of identical or similar polypeptides that are post-translationally cleaved into multiple copies of biochemically similar functional entities. The nematode polyprotein allergens/antigens (NPAs) fall into this class (Fig. 16.1). 

Biosynthesis as a tandemly repetitive polypeptide which is cleaved into multiple functionally similar proteins is highly unusual, and the only similar examples are the filaggrins of the skin of mammals (Rothnagel and Steinert, 1990). The reason why NPAs and filaggrins are produced in this way remains obscure. If a protein needs to be produced in large quantities rapidly, then a polyprotein would seem advantageous, but it seems inefficient for a genome to encode multiple copies of a protein when one will do, and other abundantly produced proteins are produced in the conventional... 

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