1.13.4S
THE ROLE OF BUCHNERA GroEL IN DETERMINING THE PERSISTENT NATURE OF PLANT VIRUSES WHICH ARE TRANSMITTED BY APHIDS IN A CIRCULATIVE, NON-PROPAGATIVE MANNER

JFJM VAN DEN HEUVEL1, SA HOGENHOUT1, M VERBEEK1, V BRAULT2, V ZIEGLER-GRAFF2, K RICHARDS2 and F VAN DER WILK1

1DLO Research Institute for Plant Pathology, PO Box 9060, 6700 GW Wageningen, The Netherlands; 2lnstitut de Biologie Moleculaire des Plantes du CNRS et de I'Universite Louis Pasteur, Strasbourg, France

Background and objectives
Luteoviruses and pea enation mosaic enamovirus (PEMV) are transmitted by aphids in a circulative, non-replicative manner. This requires virus particles to pass through the epithelial cell linings of the digestive tract and the accessory salivary glands, and to resist the potentially hostile environment of the aphid haemolymph [1, 2]. Acquired virus particles persist for several weeks in the haemolymph in which a GroEL homologue, produced by the primary bacterial endosymbiont of the aphid (a Buchnera sp.), is abundantly present [1]. We have investigated the role of Buchnera GroEL in the transmission process of Iuteoviruses, and determined the molecular basis of the interaction between luteoviruses and GroEL homologues.

Results and conclusions
The observation that both subgroup I and II luteoviruses, and PEMV, display a specific but differential affinity for GroEL homologues from the endosymbiotic bacteria of aphids [2, 3] suggests that the basic GroEL-binding capacity resides in a highly conserved region of one of the viral-capsid associated proteins. Purified luteovirus and enamovirus particles contain a major ca 22-kDa coat protein (CP), and lesser amounts of a ca 52-kDa readthrough protein (RTD), expressed by translational readthrough of the CP into the adjacent open reading frame. The RTD is accessible on the surface of a luteovirus [4]. Beet western yellows luteovirus (BWYV) mutants devoid of the RTD did not bind to Buchnera GroEL, demonstrating that the RTD, and not the highly conserved CP, contains the determinant(s) for GroEL-binding [2]. Sequence comparison of the RTDs of different luteoviruses and PEMV identified conserved residues in the N-terminal domain of the RTD potentially important in the interaction with Buchnera GroEL.

ln vivo studies showed that virions of BWYV mutants lacking the RTD were significantly less persistent in the aphid haemolymph than were virions containing the RTD. Previously we have demonstrated that antibiotic treatment of Myzus persicae larvae, which dramatically lowered the Buchnera GroEL level in the haemolymph, coincided with loss of capsid integrity of potato leafroll luteovirus (PLRV) in the haemolymph and inhibited transmissibility of this virus [1]. These data strongly suggest that the Buchnera GroEL-RTD interaction protects the virus from rapid degradation in the haemolymph of an aphid and determines the persistent nature of luteoviruses and PEMV in aphids.

Native Buchnera GroEL is, like E. coli GroEL, an oligomer of ca 840 kDa consisting of 14 identical subunits of 60 kDa arranged into two stacked heptameric rings [2]. To gain more knowledge on the nature of the association between PLRV and GroEL, the groE operon of the primary endosymbiont of M. persicae was characterized, and the PLRV-binding domain on Buchnera GroEL was identified by deletion mutant analysis [5]. This revealed that the PLRV-binding site was located in the so-called equatorial domain. Buchnera GroEL mutants lacking the entire equatorial domain, or parts of it, lost the ability to bind PLRV. The equatorial domain is made up of two regions at the N and C termini that are not contiguous in the amino-acid sequence, but which are in spatial proximity after folding of the GroEL polypeptide. Both the N- and C-terminal region of the equatorial domain were implicated in virus binding [5].

References
1. Van den Heuvel JFJM, Verbeek M, Van der Wilk F, 1994. Journal of General Virology 75, 2559-2565.
2. Van den Heuvel JFJM, Bruyere A, Hogenhout SA et al., 1997. Journal of Virology 71, 7258-7265.
3. Filichkin SA, Brumfield TP, Filichkin TP, Young MJ, 1997. Journal of Virology 71, 569-577.
4. Braijlt V, Van den Heuvel JFJM, Verbeek M et al., 1995. EMBO Journal 14, 650-659.
5. Hogenhout SA, Van der Wilk F, Verbeek M et al., 1998. Journal of Virology 72, 358-365.