The interaction of SV40 small tumor antigen with protein phosphatase 2A stimulates the map kinase pathway and induces cell proliferation.

Interaction with SV40 small tumor antigen (small t) compromised the ability of multimeric protein phosphatase 2A to inactivate the mitogen-activated protein kinase ERK1 and the mitogen-activated protein kinase kinase MEK1. Transient expression of small t in CV-1 cells activated MEK and ERK but did not affect Raf activity. Small t stimulated the growth of quiescent CV-1 cells almost as effectively as did serum. Coexpression of kinase-deficient ERK2 blocked most, but not all, of the proliferation caused by small t. Activation of the mitogen-activated protein kinase pathway and stimulation of cell growth were dependent on the interaction of small t with protein phosphatase 2A. These findings indicate that SV40 small t is capable of inducing cell growth through blockade of protein phosphatase and deregulation of the mitogen-activated protein kinase cascade.