Yeast two-hybrid systems confirm the membrane- association and oligomerization of BC1 but do not detect an interaction of the movement proteins BC1 and BV1 of Abutilon mosaic geminivirus

Most of the plant begomoviruses use two proteins to transport their DNA from cell to cell, BV1 to shuttle it between nucleus and cytoplasm and BC1 to facilitate movement across plasmodesmata. In order to analyse their interaction for Abutilon mosaic geminivirus (AbMV) in yeast (Saccharomyces cerevisiae), BC1 and BV1 genes were cloned into various plasmid vectors suitable for two-hybrid analysis. BC1 was fused to the binding domain (GBD) and BV1 to the activation domain (GAD) of the GAL4 transcription factor to check for interactions in the nucleus. Additionally, BC1 as well as BV1 were integrated into pMyr or pSos vectors to analyze protein binding at the plasma membrane using the CytoTrap™ system. Using freeze-fracture immuno-labelling (FreeFI), singly-expressed GBD:BC1 was localized at the plasma membrane although it was fused to a nuclear localization signal provided by the construct. GAD:BV1 was found in the nucleus of transformed cells as expected. Upon co-transformation of both constructs, cells grew poorly and exhibited symptoms of autolysis without any detectable level of GBD:BC1 or GAD:BV1, as revealed by FreeFI. In conclusion, both fusion proteins did not meet in the same compartment and appeared to be harmful to yeast if constitutively co-expressed. When expressed from pSos vector, BC1 induced the CytoTrap detection signal in the absence of pMyr indicating that BC1 protein alone is able to target the effector protein to the inner face of the plasma membrane. A mutated form of BC1 (ΔBC1) lacking the previously identified membrane-binding domain was no longer able to auto-induce the CytoTrap signal cascade. Using ΔBC1, an N-terminal, or a C-terminal third of BC1 revealed a homo-oligomerization of the C-terminal region of BC1 in two-hybrid analysis, but no interaction of BC1 with BV1.