Middle tumor antigen

In MTag-containing polyomaviruses, the early region contains at least three genes encoding STag, MTag, and LTag, and is transcribed as a single messenger RNA processed by alternative splicing.

Unlike STag or LTag, the MTag C-terminus contains a membrane anchor sequence that likely forms a transmembrane region.

[7] A somewhat more common tumor antigen variant, an overprinted gene encoding a protein called ALTO, may be evolutionarily related to MTag.

[1] However, MTag's best-studied functions center on its interaction with host cell proteins to activate cellular signaling pathways.

[1][4][10] Preference for members of the Src family varies, with mouse and hamster polyomavirus MTags having different distributions.

[4] Once phosphorylated, MTag interacts with and activates downstream signaling pathways through Shc, 14-3-3 proteins, phosphoinositide 3-kinase, and phospholipase Cγ1.

[1][4] The signaling functions of the phosphorylated MTag have been described as behaving like a mimic of a constitutively active receptor tyrosine kinase.

Its effectiveness in transformation is thought to be somewhat epiphenomenal to its role in the typical lytic viral life cycle.

[1][4] Because of its high efficiency as an oncovirus, particularly in newborn or immunodeficient mice, mouse polyomavirus has served as a productive mechanism for modeling tumorigenesis.

A map of the murine polyomavirus genome, indicating the early genes (LTag, MTag, and STag) at right in blue and the late genes (the viral capsid proteins) at left in red. Each region is transcribed as a single messenger RNA and alternatively spliced to express multiple proteins; the exons are shown as thickened lines. [ 2 ]