Official websites use. Share sensitive information only on official, secure websites. Middle T antigen MT is the principal oncogene of murine polyomavirus. Its study has led to the discovery of the roles tyrosine kinase and phosphoinositide 3-kinase PI3K signaling in mammalian growth control and transformation. MT is necessary for viral transformation in tissue culture cells and tumorigenesis in animals.
When expressed alone as a transgene, MT causes tumors in a wide variety of tissues. It has no known catalytic activity, but rather acts by assembling cellular signal transduction molecules. Their activation set off a series of signaling cascades. Analysis of virus mutants as well as transgenic animals has demonstrated that the effects of a given signal depend not only tissue type, but on the genetic background of the host animal.
There remain many opportunities as we seek a full molecular understanding of MT and apply some of its lessons to human cancer. Since its discovery in , polyomavirus has served as a model system for the study of transformation mechanisms and cellular signaling. Murine polyoma's role in signaling research arises from a quirk of evolution that created an extra reading frame in the early region of the virus.
The resulting third early protein was designated middle T antigen MT on the basis of its apparent molecular weight. Unlike small T ST and large T LT antigens, which work in transformation primarily by acting on host tumor suppressor proteins such as p53, pRb and protein phosphatase 2A PP2A , MT acts by activating key proto-oncogenes involved in cellular signaling.
Its primary structure is diagrammed below in figure 1. In the case of the RTK, a ligand is used to bind a second RTK, thus activating both members of the ligand induced dimer.
