Mutations in the helper component protease gene of zucchini yellow mosaic virus affect its ability to mediate aphid transmissibility

Abstract

The nucleotide sequence of the helper component protease (HC-Pro) genes of three zucchini yellow mosaic virus (ZYMV) strains has been compared with that of a helper-deficient strain of ZYMV-HC. The comparisons revealed three unique deduced amino acid differences. Two of these mutations were located in regions which are conserved in other potyviruses. The role of these mutations in aphid transmissibility was examined by exchanging DNA fragments of part of the deficient HCPro gene with the respective section within the gene of the infectious full-length clone of the aphid-transmissible ZYMV. The first exchange included two of the three mutations, the first coding for a change from Asp to Gly (in a non-conserved region) and the second coding for a change from Arg to Ile [within the Phe-Arg-Asp-Lys (FRNK) conserved box]. This exchange resulted in a reduced transmission (20-6% for the mutated virus compared with 57-4% in the normal ZYMV when acquired from plants and 37.2 % compared with 83"1%, respectively, when acquired from membranes). The second exchange incorporated a single mutation [conferring a change from Thr to Ala within the Pro-Thr-Lys (PTK) conserved box]. This single mutation resulted in almost total loss of HC activity in aphid transmission both from plants and from membranes. The Lys residue in the conserved Lys-Ile-Thr-Cys (KITC) box, which is related to loss of HC activity in potato virus Y, tobacco vein mottling virus and in the Michigan strain of ZYMV, is unchanged in the helper-deficient ZYMV. It is therefore proposed that more than one site in HC-Pro may be functionally related to aphid transmissibility. The possible reasons for the role of these mutations in helper activity in aphid transmission of ZYMV are discussed.

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

“…The N-terminal portion of HC-Pro is required for aphidmediated plant-to-plant transmission (Atreya et al, 1992;Atreya & Pirone, 1993). However, in a recent report a point mutation within the conserved C-terminal proline-threonine-lysine (PTK) box of HC-Pro was shown to be associated with loss of aphid transmissibility of zucchini yellow mosaic potyvirus (Granier et al, 1993;Huet et al, 1994), suggesting that more than one site in HC-Pro could be involved in aphid transmission activity.…”

Potyvirus helper component-proteinase and coat protein (CP) have coordinated functions in virus-host interactions and the same CP motif affects virus transmission and accumulation.

“…The S7G substitution in CP (which reduced virus accumulation) restored transmissibility to PVA-B11 (no plants infected of the 40 plants inoculated), as shown by transmission of B11-DAG (11\20) and B11-U CP (5\10) to a rate similar to PVA-U (6\10). The motifs within HC-Pro required for aphidtransmissibility in other potyviruses (Atreya & Pirone, 1993 ;Huet et al, 1994) are conserved in PVA-B11 (Puurand et al, 1994 ;Rajama$ ki et al, 1998). The requirement of the CP Nterminal DAG motif for aphid-transmissibility has been proven in four other potyviruses by mutational analysis of the icDNA clones (Atreya et al, 1991 ;Gal-On et al, 1992 ;Flasinski et al, 1996 ;Lo!…”

“…The N-terminal part of HC-Pro affects virulence (symptom severity), genome amplification and virus accumulation (Atreya et al, 1992 ;Atreya & Pirone, 1993 ;Kasschau & Carrington, 1995). KITC and PTK amino acid motifs of HC-Pro are required for transmissibility by aphid vectors (Atreya & Pirone, 1993 ;Huet et al, 1994). The central region of HC-Pro affects long-distance movement of virus (Cronin et al, 1995), whereas the C-terminal part is needed for cell-to-cell movement Author for correspondence : Jari Valkonen (at Department of Plant Biology, Uppsala).…”

Zucchini yellow mosaic virus

“…For another ZYMV isolate, two mutations were found between the HAT and PAT strains, one of them occurring in a conserved cluster of amino-acids Pro-Thr-Lys (PTK) (Granier et al, 1993). Huet et al (1994) modified the PTK of ZYMV to PAK in an infectious cDNA clone and observed a total loss of HC activity in aphid transmission. An aminoacid exchange (R to I) in another conserved box (the FRNK box) resulted in more than 50% reduction in aphid transmission, but did not completely abolish transmission (Huet et al, 1994).…”

“…Huet et al (1994) modified the PTK of ZYMV to PAK in an infectious cDNA clone and observed a total loss of HC activity in aphid transmission. An aminoacid exchange (R to I) in another conserved box (the FRNK box) resulted in more than 50% reduction in aphid transmission, but did not completely abolish transmission (Huet et al, 1994).…”