Plant Viral Proteases: Beyond the Role of Peptide Cutters

Abstract

Almost half of known plant viral species rely on proteolytic cleavages as key co- and post-translational modifications throughout their infection cycle. Most of these viruses encode their own endopeptidases, proteases with high substrate specificity that internally cleave large polyprotein precursors for the release of functional sub-units. Processing of the polyprotein, however, is not an all-or-nothing process in which endopeptidases act as simple peptide cutters. On the contrary, spatial-temporal modulation of these polyprotein cleavage events is crucial for a successful viral infection. In this way, the processing of the polyprotein coordinates viral replication, assembly and movement, and has significant impact on pathogen fitness and virulence. In this mini-review, we give an overview of plant viral proteases emphasizing their importance during viral infections and the varied functionalities that result from their proteolytic activities.

Citrus Psorosis Virus Movement Protein Contains an Aspartic Protease Required for Autocleavage and the Formation of Tubule-Like Structures at Plasmodesmata

“…Both in transientexpression experiments as well as during infection, we see that the processing of full-length MP CPsV is not exhaustive, and a high proportion of MP CPsV remains full length. This is also observed for the trans-cleavage of MP CPsV D340A by 20K CPsV , suggesting that the protease activity is under regulation (60). Mutants at the cleavage site evaluated in this work showed the capacity to undergo autocleavage, even when the amino acids 310 LA 311 were replaced with residues with different physicochemical properties, suggesting that these positions are not critical for recognition of the cleavage site.…”

“…Based on the importance of the 34K fragment of MP CPsV for virus movement, it may be expected that this fragment interacts with CP within the tubules. Plant virus-encoded proteases belong to three classes (60). Whereas serine and serine-like proteases have been described in the Potyviridae (61,62) and Secoviridae (63)(64)(65) families, cysteine proteases are present in Beny- (66), Marafi-(67), Tymo- (68), and Closterovirus (69), while aspartic proteases occur in the Caulimoviridae family (54,70).…”

Strawberry Mottle Virus (Family Secoviridae , Order Picornavirales ) Encodes a Novel Glutamic Protease To Process the RNA2 Polyprotein at Two Cleavage Sites

“…For example, pestivirus polyproteins are cleaved by three viral proteases and two host proteases (the signal peptidase and signal peptide peptidase) (1). Viral proteases are often multifunctional and play key roles in various aspects of the virus infection cycle in addition to the processing of viral polyproteins (2). For example, some viral proteases cleave host proteins to facilitate virus translation or replication or to downregulate host defense responses, as exemplified by the picornavirus 2A and 3C proteases (3)(4)(5).…”

“…All characterized proteases encoded by positive-strand RNA viruses are cysteine or serine proteases, most of which are structurally related to the cellular papain cysteine protease, trypsin serine protease, and chymotrypsin serine protease (2). The picornavirus 3C proteases and related 3C-like proteases (3CL-Pro) are cysteine proteases that adopt a chymotrypsin-like structure (10).…”

“…It is also possible that Pro2-Glu has other biological functions in addition to its processing activity. Indeed, many plant virus proteases are multifunctional, playing additional role in pathogenicity, RNA silencing suppression, or vector transmission (2). Further work will be aimed at investigating these possibilities.…”