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Updated: May 30, 2026
Electrophoretic Analysis of Replication Through Structure-Prone DNA Repeats Within the SV40-Based Human Episome
Published on: September 13, 2024
Comparisons between murine polyomavirus and Simian virus 40 show significant differences in small T antigen function
Shaida Andrabi1, Justin H Hwang, Jennifer Kean Choe
1Department of Biochemistry, Tufts University School of Medicine, Boston, MA 02111, USA.
Abstract:
Although members of a virus family produce similar gene products, those products may have quite different functions. Simian virus 40 (SV40) large T antigen (LT), for example, targets p53 directly, but murine polyomavirus LT does not. SV40 small T antigen (SVST) has received considerable attention because of its ability to contribute to transformation of human cells. Here, we show that there are major differences between SVST and polyomavirus small T antigen (POLST) in their effects on differentiation, transformation, and cell survival. Both SVST and POLST induce cell cycle progression. However, POLST also inhibits differentiation of 3T3-L1 preadipocytes and C2C12 myoblasts. Additionally, POLST induces apoptosis of mouse embryo fibroblasts. SVST reduces the proapoptotic transcriptional activity of FOXO1 through phosphorylation. On the other hand, SVST complements large T antigen and Ras for the transformation of human mammary epithelial cells (HMECs), but POLST does not. Mechanistically, the differences between SVST and POLST may lie in utilization of protein phosphatase 2A (PP2A). POLST binds both Aα and Aβ scaffolding subunits of PP2A while SVST binds only Aα. Knockdown of Aβ could mimic POLST-induced apoptosis. The two small T antigens can target different proteins for dephosphorylation. POLST binds and dephosphorylates substrates, such as lipins, that SVST does not.
Insights
Simian virus 40 small T antigen (SVST) and polyomavirus small T antigen (POLST) exhibit distinct functions in cell differentiation, survival, and transformation. SVST promotes cell cycle progression and transformation, while POLST inhibits differentiation and induces apoptosis.
Area of Science:
- Virology
- Molecular Biology
- Cell Biology
Background:
- Viral oncoproteins, like Simian virus 40 large T antigen (LT), can have diverse functions despite belonging to the same virus family.
- SV40 small T antigen (SVST) is known for its role in human cell transformation.
- Significant functional differences exist between SVST and polyomavirus small T antigen (POLST).
Purpose of the Study:
- To investigate and delineate the distinct effects of SVST and POLST on cellular differentiation, transformation, and survival.
- To elucidate the molecular mechanisms underlying the functional divergence between SVST and POLST.
- To compare the interactions of SVST and POLST with protein phosphatase 2A (PP2A) and its subunits.
Main Methods:
- Comparative analysis of SVST and POLST functions in cell cycle progression, differentiation, and apoptosis.
- Assessment of SVST and POLST roles in the transformation of human mammary epithelial cells (HMECs) in conjunction with LT and Ras.
- Investigation of SVST and POLST binding affinities and interactions with protein phosphatase 2A (PP2A) scaffolding subunits (Aα and Aβ).
- Functional analysis of PP2A subunit Aβ knockdown to mimic POLST-induced apoptosis.
- Identification of distinct protein substrates dephosphorylated by SVST and POLST, including lipins.
Main Results:
- Both SVST and POLST promote cell cycle progression.
- POLST inhibits differentiation in 3T3-L1 preadipocytes and C2C12 myoblasts, whereas SVST does not.
- POLST induces apoptosis in mouse embryo fibroblasts, while SVST reduces FOXO1 proapoptotic activity.
- SVST collaborates with LT and Ras for HMEC transformation, but POLST does not.
- POLST binds both Aα and Aβ subunits of PP2A, while SVST binds only Aα.
- Knockdown of Aβ mimics POLST-induced apoptosis, suggesting its critical role.
- POLST dephosphorylates substrates like lipins, which are not targeted by SVST.
Conclusions:
- SVST and POLST exhibit significant functional divergence in cellular processes, including differentiation, survival, and transformation.
- Differential interaction with PP2A subunits, particularly Aβ, underlies key functional differences, including POLST-induced apoptosis.
- The distinct substrate specificities for dephosphorylation contribute to the varied biological outcomes mediated by SVST and POLST.
