Cauliflower mosaic virus gene I product (P1) forms tubular structures which extend from the surface of infected protoplasts.

Immunofluorescent staining of Chinese cabbage protoplasts infected with cauliflower mosaic virus (CaMV) showed different patterns of staining when antibodies raised against virus particles or antibodies specific for the CaMV movement protein (P1) were used. Anti-virion serum showed that particles accumulated as small discrete aggregates within the cytoplasm of infected protoplasts. In contrast, anti-P1 serum identified long thread-like structures extending from the protoplast surface. The same structures were seen when protoplasts were isolated from infected plants and held in culture. Electron microscopical examination after immunogold labeling and negative staining of protoplasts revealed some extracellular tubular structures which labeled with anti-P1 and which contained virus particles. Western blot analysis of isolated tubules showed them to be composed of the full-length P1 but not the smaller P1 products detected in infected tissues or protoplasts. The time course of P1 accumulation in infected protoplasts paralleled the accumulation of virus coat protein, as measured by immunofluorescence microscopy, and did not indicate that P1 was an early gene product as has been suggested for other virus movement proteins.