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Aphids, viruses and the cytoskeleton Blanc, S., Schmidt, I., Vantard, M., Scholthof, H.B., Kuhl, G., Esperandieu, P., Cerutti, M. and Louis, C. (1996) The aphid transmisSion factor of cauliflower mosaic virus forms a stable complex with microtubules in both insect and plant cells, Proc. Natl. Acad. Sci. U. S. A. 93, 15158-15163 Aphids play an important part in the transmission of viruses between host plants. Blanc et al. have examined how viruses are retained within aphid feeding apparatus by expressing the 'aphid transmission factor' of cauliflower mosaic virus (CaMV) in a baculovirus recombinant insect cell line. The transmission factor is encoded by CaMV gene II, which gives rise to a polypeptide of 18 kDa (plS). This polypeptide does not appear to be necessary for virus replication or cell-to-cell movement, but does interact with virus particles and aphid stylets. When expressed in insect cells, p18 induces abnormal features in the cell cytoskeleton, resulting in linear microtubule-like structures throughout the cytoplasm. These structures are decorated with paracrystals characteristic of p18. A biologically inactive p18 mutant (p18 157M),
which contains a point mutation preventing it from binding viral particles, was unable to form paracrystals, but was able to induce the formation of unusual cytoskeletal structures - these were observed as a net-like sheath that coated microtubules. Both wild-type and mutant p18 had the effect of stabilizing the microtubules, rendering the pl8-microtubule complex more resistant to conditions that promote microtubule disassembly. Transient transfection experiments in protoplasts of tobacco confirmed the observations on insect cells, with apparent colocalization of p18 and the microtubule network in vivo. The association between p18 and microtubules occurred even at high ionic strength - conditions under which microtubuleassociated proteins (MAPs) dissociate. This suggests that p18 binds directly to microtubules and not via a MAP.
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La Roche, J., Van Der Staay, G.W.M., Partensky, F., Ducret, A., Aebersold, R., Li, g., Golden, S.S., Hiller, R.G., Wrench, P.M., Larkum, A.W.B. and Green, s.g. (1996) Independent evolution of the prochlorophyte and green plant chlorophyll a/b light-harvesting proteins, Proc. Natl. Acad. Sci. U. S. A. 93, 15244-15248 Prochlorophytes are prokaryotes that carry out oxygenic photosynthesis using chlorophylls (Chl) a and b, and unlike other cyanobacteria they lack the light-harvesting phycobilins. This has led to a debate as to whether the chloroplasts of green algae have arisen from an endosymbiotic prochlorophyte rather than a cyanobacterium. It had previously been shown that the prochlorophyte Chl a/b binding proteins are immunologically related to each other, but there was no apparent cross-reactivity with higher plant light-harvesting complexes. Here, the authors have examined the origin of prochlorophyte Chl a/b binding proteins by isolating the genes encoding them from members of all three prochlorophyte genera. Sequence analysis revealed that the cloned genes are related to a cyanobacterial gene (isiA) encoding a Chl a binding protein that is induced by iron starvation. However, none of the prochlorophyte sequences have homology to members of the eukaryotic Chl
© 1997 Elsevier Science Ltd
a/b antenna family. The prochlorophyte Chl a/b binding proteins and cyanobacterial IsiA sequences share approximately 54% identity. Phylogenetic trees based on the protein sequences deduced from the cloned prochlorophyte genes, IsiA sequences and several core Chl a sequences revealed that the IsiA proteins duster in a well-supported dude, but that the prochlorophyte proteins form a much weaker group. The findings support an evolutionary pathway in which the ability to synthesize Chl b may have arisen several times by gene duplication and divergence. Thus, the presence of Chl a/b light-harvesting proteins in both prochlorophytes and green chloroplasts appears to reflect convergent evolution. ;
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April 1997,Voi. 2, No. 4
1 23
literature
news
focus
Aphids, viruses and the cytoskeleton Blanc, S., Schmidt, I., Vantard, M., Scholthof, H.B., Kuhl, G., Esperandieu, P., Cerutti, M. and Louis, C. (1996) The aphid transmisSion factor of cauliflower mosaic virus forms a stable complex with microtubules in both insect and plant cells, Proc. Natl. Acad. Sci. U. S. A. 93, 15158-15163 Aphids play an important part in the transmission of viruses between host plants. Blanc et al. have examined how viruses are retained within aphid feeding apparatus by expressing the 'aphid transmission factor' of cauliflower mosaic virus (CaMV) in a baculovirus recombinant insect cell line. The transmission factor is encoded by CaMV gene II, which gives rise to a polypeptide of 18 kDa (plS). This polypeptide does not appear to be necessary for virus replication or cell-to-cell movement, but does interact with virus particles and aphid stylets. When expressed in insect cells, p18 induces abnormal features in the cell cytoskeleton, resulting in linear microtubule-like structures throughout the cytoplasm. These structures are decorated with paracrystals characteristic of p18. A biologically inactive p18 mutant (p18 157M),
which contains a point mutation preventing it from binding viral particles, was unable to form paracrystals, but was able to induce the formation of unusual cytoskeletal structures - these were observed as a net-like sheath that coated microtubules. Both wild-type and mutant p18 had the effect of stabilizing the microtubules, rendering the pl8-microtubule complex more resistant to conditions that promote microtubule disassembly. Transient transfection experiments in protoplasts of tobacco confirmed the observations on insect cells, with apparent colocalization of p18 and the microtubule network in vivo. The association between p18 and microtubules occurred even at high ionic strength - conditions under which microtubuleassociated proteins (MAPs) dissociate. This suggests that p18 binds directly to microtubules and not via a MAP.
op
La Roche, J., Van Der Staay, G.W.M., Partensky, F., Ducret, A., Aebersold, R., Li, g., Golden, S.S., Hiller, R.G., Wrench, P.M., Larkum, A.W.B. and Green, s.g. (1996) Independent evolution of the prochlorophyte and green plant chlorophyll a/b light-harvesting proteins, Proc. Natl. Acad. Sci. U. S. A. 93, 15244-15248 Prochlorophytes are prokaryotes that carry out oxygenic photosynthesis using chlorophylls (Chl) a and b, and unlike other cyanobacteria they lack the light-harvesting phycobilins. This has led to a debate as to whether the chloroplasts of green algae have arisen from an endosymbiotic prochlorophyte rather than a cyanobacterium. It had previously been shown that the prochlorophyte Chl a/b binding proteins are immunologically related to each other, but there was no apparent cross-reactivity with higher plant light-harvesting complexes. Here, the authors have examined the origin of prochlorophyte Chl a/b binding proteins by isolating the genes encoding them from members of all three prochlorophyte genera. Sequence analysis revealed that the cloned genes are related to a cyanobacterial gene (isiA) encoding a Chl a binding protein that is induced by iron starvation. However, none of the prochlorophyte sequences have homology to members of the eukaryotic Chl
© 1997 Elsevier Science Ltd
a/b antenna family. The prochlorophyte Chl a/b binding proteins and cyanobacterial IsiA sequences share approximately 54% identity. Phylogenetic trees based on the protein sequences deduced from the cloned prochlorophyte genes, IsiA sequences and several core Chl a sequences revealed that the IsiA proteins duster in a well-supported dude, but that the prochlorophyte proteins form a much weaker group. The findings support an evolutionary pathway in which the ability to synthesize Chl b may have arisen several times by gene duplication and divergence. Thus, the presence of Chl a/b light-harvesting proteins in both prochlorophytes and green chloroplasts appears to reflect convergent evolution. ;
=
6e
;that ~ach ~i6l# of the op#~:~e2gen6 (5~th
:::!
Evolution of chloroph}all-binding proteins
exa
gel
~: ( {
GE);:
Thi/~ihe~
:
char~cteriS~!~ ~f i~h~ph~ 5~: ::: ~ation 6f Opaque2~ith p ac!~ ~ ~d, ~ th~ app ce:of,~ fo electric focu~ii~: :Th~ ~!g~fi~
~ed: ~iS OpaqUe2: ~ : i : c h ~ i &iiP ~ la oi ci~d~aini~:i~: ze~ pr0n~o~e~~ d~ ~:
=
transcription rate~ ~ ~ow
:aVait~:
April 1997,Voi. 2, No. 4
1 23