Viral protein R (Vpr) of human immunodeficiency virus type 1 inhibits cell proliferation by arresting the cell cycle at the G2 phase and inducing to apoptosis after G2 arrest. We have reported previously that C81, a carboxy-terminally truncated form of Vpr, interferes with cell proliferation via a novel pathway that is distinct from G2 arrest. However, the mechanism of this effect of C81 is unknown. We demonstrate here that C81 can induce apoptosis via G1 arrest of the cell cycle. Immunostaining for various markers of stages of the cell cycle and flow cytometry analysis of DNA content showed that most HeLa cells that had been transiently transfected with a C81 expression vector were arrested at the G1 phase and not at the G2 or S phase of the cell cycle. Staining for annexin V, which binds phosphatidylserine on the plasma membrane, as an early indicator of apoptosis and measurement of the activity of caspase-3, a signaling molecule in apoptotic pathways, indicated that C81 is a strong inducer of apoptosis. Expression of C81 induced the condensation, fragmentation, and clumping of chromatin that are typical of apoptosis. Furthermore, the kinetics of the C81-induced G1 arrest were closely correlated with changes in the number of annexin V-positive cells and the activity of caspase-3. Replacement of Ile or Leu residues by Pro at positions 60, 67, 74, and 81 within the leucine zipper-like domain of C81 revealed that Ile60, Leu67, and Ile74 play important roles both in the C81-induced G1 arrest and in apoptosis. Thus, it appears that C81 induces apoptosis through pathways that are identical to those utilized for G1 arrest of the cell cycle. It has been reported that Ile60, Leu67, and Ile74 also play an important role in the C81-induced suppression of growth. These results suggest that the suppression of growth induced by C81 result in apoptosis that is independent of G2 arrest of the cell cycle.