RESUMO
Phytohemagglutinin (PHA) is known to increase the synthesis of bovine leukemia virus (BLV) particles and viral antigens in short-term culture of BLV-infected neoplastic and non-neoplastic lymphocytes. This stimulation of BLV expression has been shown to be due to enhanced transcription of the viral genome by a PHA-induced protein. We have investigated the binding of 125I-labelled PHA to BLV-infected bovine B lymphocytes and subsequent events that may lead to the stimulation of BLV-p25 synthesis. We found that PHA binding to the infected cells were rapid, but only a small fraction of the bound PHA is translocated to nucleus. However, bound PHA dissociated rapidly from the cell membrane, but not from the nucleus when PHA is removed from the culture medium. Furthermore, continuous presence of PHA was not essential for optimal stimulatory activity, instead a minimum incubation with PHA for 6 hr. followed by culturing the infected cells in its absence, was sufficient to exhibit maximal stimulatory activity. Our results raise the possibility that interaction of PHA with the plasma membrane probably triggers synthesis of a protein which in turn enhances the transcription of BLV genome in vitro.