Long-Distance Movement Factor: A Transport Function of the Potyvirus Helper Component Proteinase

Abstract

Transport of viruses from cell to cell in plants typically involves one or more viral proteins that supply dedicated movement functions. Transport from leaf to leaf through phloem, or long-distance transport, is a poorly understood process with requirements differing from those of cell-to-cell movement. Through genetic analysis of tobacco etch virus (TEV; potyvirus group), a novel long-distance movement factor was identified that facilitates vascular-associated movement in tobacco. A mutation in the central region of the helper component proteinase (HC-Pro), a TEV-encoded protein with previously described activities in aphid-mediated transmission and polyprotein processing, inactivated long-distance movement. This mutant virus exhibited only minor defects in genome amplification and cell-to-cell movement functions. In situ histochemical analysis revealed that the mutant was capable of infecting mesophyll, bundle sheath, and phloem cells within inoculated leaves, suggesting that the long-distance movement block was associated with entry into or exit from sieve elements. The long-distance movement defect was specifically complemented by HC-Pro supplied in trans by a transgenic host. The data indicate that HC-Pro functions in one or more steps unique to long-distance transport.

Cloning of the Arabidopsis RTM1 gene, which controls restriction of long-distance movement of tobacco etch virus

“…First, RTM1, in cooperation with other factors, could block long-distance movement of TEV by preventing entry into, passage through, or exit from the phloem. This could occur if RTM1, or a RTM1-containing complex, physically interacts with one or more of the TEV factors required for long-distance movement (HC-Pro, CI, NIa, and capsid) (34)(35)(36)(37)(38)(39). The RTM1 protein or complex also could inhibit the function of cellular factors or structures required for long-distance movement.…”

Nucleic acid-binding properties of a bacterially expressed potato virus Y helper component-proteinase

“…Interestingly, the mutation mapped in the HC-Pro gene, suggesting an additional putative function for this protein. The functional importance of HC-Pro in systemic movement was further supported by Cronin et al (1995) using a movement-defective tobacco etch potyvirus (TEV) containing a mutation in the central region of HC-Pro, complemented by transgenic plants expressing the functional HC-Pro.…”

“…Although interaction between HC-Pro and the virus coat protein (CP) has been proposed as an overall molecular model to explain some of the biological functions described (Pirone, 1991;Cronin et al, 1995), two lines of evidence suggest that HC-Pro also possesses a nucleic acid-binding activity that is related to its multifunctionality. HC-Pro purified from infected tobacco plants has affinity for homoribopolymers such as poly(U), poly(G) and poly(I) but not for poly(A) or poly(C) (Thornbury et aI., 1985).…”

Cell-to-cell movement of three genera (+) ss RNA plant viruses

“…Some genetic analyses indicate the involvement of the HC-Pro protein in cell-to-cell transport, but its direct role in this process is not clear (Cronin et al 1995;Kasschau et al 1997). The TEV HC-Pro mutant appeared to move from cell-to-cell less efficiently than the native/natural virus (Kasschau et al 1997), whereas TVMV HC-Pro mutant was unable to spread in inoculated leaves (Klein et al 1994).…”