Summary. In order to monitor their interaction and cellular localisation, the movement protein (MP; syn. BC1) and the nuclear shuttle protein (NSP; syn. BV1) of the geminivirus Abutilon mosaic virus (AbMV) were ectopically expressed in Schizosaccharomyces pombe cells, either alone or together under the control of an inducible promoter. For highest resolution, electron microscopy using freeze-fracture immunolabelling served to detect these proteins in situ. As expected from previous in planta and yeast experiments, NSP accumulated within the nuclei, whereas MP was targetted to the protoplasmic face of plasma mem- branes when expressed alone. Upon coexpression, NSP was localised at the plasma membranes, where it was strongly attached. These results support a model in which NSP transports viral DNA to the cell periphery to facilitate cell-to-cell movement of viral DNA within plants. In con- trast to AbMV MP, no plant-specific protein seems to be necessary for the translocation of NSP to the plasma membrane. Keywords: Schizosaccharomyces pombe; Freeze-fracture immunola- belling; Protein expression. Introduction Plant viruses use specific movement proteins (MPs) to fa- cilitate symplastic transport of their nucleic acids via plas- modesmata (Waigmann et al. 2004). In contrast to most RNA plant viruses, the single-stranded-DNA-containing geminiviruses replicate within nuclei and, therefore, have to cross an additional border, the nuclear envelope, to in- fect a plant systemically (Gafni and Epel 2002). The mem- bers of the family Geminiviridae have evolved several mechanisms to overcome this hurdle. Most members of the genus Begomovirus possess a bipartite genome con- sisting of DNA A and DNA B and use the cooperation of two genes on DNA B, called MP and NSP, to spread the virus within the plant (Lazarowitz and Beachy 1999). Over the past decade, a scientific consensus has grown that MP and NSP cooperate for cell-to-cell movement of viral DNA, NSP is acting as nuclear shuttle protein (NSP) and MP as movement protein (MP) in a narrow sense. However, two models describing their interactions have been proposed. According to the “relay race model”, NSP shuttles viral DNA from the nucleus to the cytoplasm, where DNA is passed to MP for transport through plas- modesmata (Gilbertson et al. 2003; Noueiry et al. 1994; Rojas et al. 1998, 2001). Alternatively, according to the “double-skating model”, the NSP–viral DNA complex is exported from the nucleus and, provided with a membrane adapter (MP), transferred to the next cell (Hehnle et al. 2004, Ingham et al. 1995, Pascal et al. 1994, Qin et al. 1998, Sanderfoot et al. 1996, Sanderfoot and Lazarowitz 1995, Ward and Lazarowitz 1999, Ward et al. 1997). Res- olution of the actual mechanism remains to be achieved and may not be possible since the two models are based on studies by different techniques and with different viruses which may have evolved differing mechanisms. Our own work on Abutilon mosaic virus (AbMV) supports the sec- ond transport model. (Abutilon mosaic virus is a species in the genus Begomovirus, family Geminiviridae.) AbMV The movement protein BC1 promotes redirection of the nuclear shuttle protein BV1 of Abutilon mosaic geminivirus to the plasma membrane in fission yeast Short communication S. Frischmuth 1 , C. Wege 1 , D. Hülser 2 , and H. Jeske 1, * 1 Department of Molecular Biology and Plant Virology, Institute of Biology, University of Stuttgart, Stuttgart 2 Department of Biophysics, Institute of Biology, University of Stuttgart, Stuttgart Received February 23, 2006; accepted April 7, 2006; published online February 19, 2007 © Springer-Verlag 2007 * Correspondence and reprints: Department of Molecular Biology and Plant Virology, Institute of Biology, University of Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart, Germany. E-mail: holger.jeske@bio.uni-stuttgart.de Protoplasma (2007) 230: 117–123 DOI 10.1007/s00709-006-0223-x PROTOPLASMA Printed in Austria