The human cytomegalovirus UL82 gene product (pp71) accelerates progression through the G₁ phase of the cell cycle

As viruses are reliant upon their host cell to serve as proper environments for their replication, many have evolved mechanisms to alter intracellular conditions to suit their own needs. For example, human cytomegalovirus induces quiescent cells to enter the cell cycle and then arrests them in late G₁, before they enter the S phase, a cell cycle compartment that is presumably favorable for viral replication. Here we show that the protein product of the human cytomegalovirus UL82 gene, pp71, can accelerate the movement of cells through the G₁ phase of the cell cycle. This activity would help infected cells reach the late G₁ arrest point sooner and thus may stimulate the infectious cycle. pp71 also induces DNA synthesis in quiescent cells, but a pp71 mutant protein that is unable to induce quiescent cells to enter the cell cycle still retains the ability to accelerate the G₁ phase. Thus, the mechanism through which pp71 accelerates G₁ cell cycle progression appears to be distinct from the one that it employs to induce quiescent cells to exit G₀ and subsequently enter the S phase.