Biochemical characterization of the helper component of Cauliflower mosaicvirus

The helper component of Cauliflower mosaic virus is encoded by viral gene I I. This protein (P2) is dispensable for virus replication but required for aphid transmission. The purification of P2 has never been reported, and hen ce its biochemical properties are largely unknown. We produced the P2 prote in via a recombinant baculovirus with a His tag fused at the N terminus. Th e fusion protein was purified by affinity chromatography in a soluble and b iologically active form. Matrix-assisted laser desorption time-of-flight ma ss spectrometry demonstrated that P2 is not posttranslationally modified. U V circular dichroism revealed the secondary structure of P2 to be 23% alpha -helical. Most alpha -helices are suggested to be located in the C-termina l domain. Using size exclusion chromatography and aphid transmission testin g, we established that the active form of P2 assembles as a huge soluble ol igomer containing 200 to 300 subunits. We further showed that P2 can also p olymerize as long paracrystalline filaments. We mapped P2 domains involved in P2 self-interaction, presumably through coiled-coil structures, one of w hich is proposed to form a parallel trimer. These regions have previously b een reported to also interact with viral P3, another protein involved in ap hid transmission. Possible interference between the two types of interactio n is discussed with regard to the biological activity of P2.