A 5′-proximal region of the Citrus tristeza virus genome encoding two leader proteases is involved in virus superinfection exclusion

Abstract

Superinfection exclusion (SIE), a phenomenon in which a primary virus infection prevents a secondary infection with the same or closely related virus, has been observed with various viruses. Earlier we demonstrated that SIE by Citrus tristeza virus (CTV) requires viral p33 protein. In this work we show that p33 alone is not sufficient for virus exclusion. To define the additional viral components that are involved in this phenomenon, we engineered a hybrid virus in which a 5'-proximal region in the genome of the T36 isolate containing coding sequences for the two leader proteases L1 and L2 has been substituted with a corresponding region from the genome of a heterologous T68-1 isolate. Sequential inoculation of plants pre-infected with the CTV L1L2T68 hybrid with T36 CTV resulted in superinfection with the challenge virus, which indicated that the substitution of the L1-L2 coding region affected SIE ability of the virus.

The Coat Protein and NIa Protease of Two Potyviridae Family Members Independently Confer Superinfection Exclusion

“…Taken together, we mapped the effectors of superinfection exclusion encoded by WSMV and TriMV through gain-of-function assays. In contrast, investigations with CTV identified p33 and L1L2 proteinases as determinants of SIE through loss-of-function (49,54). CTV-encoded p33 was found to be a determinant of SIE since CTV lacking the p33 ORF or with heterologous p33 failed to trigger SIE against identical superinfectors (49).…”

“…CTV-encoded p33 was found to be a determinant of SIE since CTV lacking the p33 ORF or with heterologous p33 failed to trigger SIE against identical superinfectors (49). Additionally, CTV L1L2 from the heterologous T68 strain failed to elicit SIE, thus confirming the requirement of L1L2 for CTV SIE (54). Yet, neither of the CTV proteins was able to effect SIE when expressed heterologously.…”

“…Furthermore, WSMV NIa-Pro deletion analyses revealed that the sequence encoding complete NIa-Pro is required for SIE activity, thus suggesting that protease activity is essential to induce SIE. Recently, L1L2 proteases of CTV were also implicated in SIE as the primary virus with a heterologous leader protease sequence facilitating efficient superinfection by the challenge virus (54). However, the exact mechanisms of plant viral proteases in SIE need further research.…”

“…Recently, the gene products of p33 and L1L2 of CTV were demonstrated to be required for SIE, but substitution with a heterologous p33 or L1L2 gene was not sufficient for SIE of either homologous or heterologous virus (49,54), indicating that other gene products must be involved. Coat protein (CP) and HC-Pro of PVA have been implicated in cross-protection, as PVA with heterologous CP or HC-Pro cistrons failed to cross-protect the homologous strains (22).…”

A self-perpetuating repressive state of a viral replication protein blocks superinfection by the same virus

“…Studies using CTV (different from TCV!) found that SIE can be abolished by replacing three virus-encoded proteins, namely the replication-related L1 and L2 proteases, as well as movement-related p33, with their counterparts from a distantly related CTV strain [8, 31]. Similarly, SIE of WNV and HCV was found to be reversed by mutations in a few viral proteins, among them WNV-encoded 2K and NS4A, and HCV-encoded E1, p7, and NS5A [4, 19].…”