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Updated: Apr 24, 2026
Merkel Cell Polyomavirus Infection and Detection
Published on: February 7, 2019
Characterization of functional domains in the Merkel cell polyoma virus Large T antigen
Roland Houben1, Sabrina Angermeyer, Sebastian Haferkamp
1Department of Dermatology, University Hospital, Würzburg, Germany.
Abstract:
Merkel cell polyomavirus (MCPyV)--positive Merkel cell carcinoma (MCC) tumor cell growth is dependent on the expression of a viral Large T antigen (LT) with an intact retinoblastoma protein (RB)-binding site. This RB-binding domain in MCPyV-LT is--in contrast to other polyomavirus LTs (e.g., SV40)--embedded between two large MCPyV unique regions (MUR1 and MUR2). To identify elements of the MCPyV-LT necessary for tumor cell growth, we analyzed the rescue activity of LT variants following knockdown of the endogenous LT in MCC cells. These experiments demonstrate that nuclear localization is essential for LT function, but that a motif previously described to be a nuclear localization sequence is neither required for nuclear accumulation of truncated MCPyV-LT nor for promotion of MCC cell proliferation. Furthermore, large parts of the MURs distal to the RB binding domain as well as ALTO--a second protein encoded by an alternative reading frame in the MCPyV-LT mRNA--are completely dispensable for MCPyV-driven tumor cell proliferation. Notably, even MCPyV-LTs in which the entire MURs have been removed are still able to promote MCC cellular growth although rescue activity is reduced which may be due to MUR1 being required for stable LT expression in MCC cells. Finally, we provide evidence implying that--while binding to Vam6p is not essential--HSC-70 interaction is significantly involved in mediating MCPyV-LT function in MCC cells including growth promotion and induction of E2F target genes.
Insights
Merkel cell polyomavirus Large T antigen (LT) drives Merkel cell carcinoma growth by binding retinoblastoma protein (RB). Key regions and nuclear localization are essential, but some viral domains are dispensable for proliferation.
Area of Science:
- Oncology
- Virology
- Molecular Biology
Background:
- Merkel cell carcinoma (MCC) is a rare skin cancer often driven by Merkel cell polyomavirus (MCPyV).
- MCPyV Large T antigen (LT) expression is crucial for MCC tumor cell proliferation.
- The RB-binding site within MCPyV LT is essential, flanked by unique regions (MUR1, MUR2).
Purpose of the Study:
- To identify essential elements of MCPyV LT required for MCC tumor cell growth.
- To elucidate the functional roles of specific MCPyV LT domains and interactions in viral oncogenesis.
Main Methods:
- Analysis of LT variants' rescue activity after endogenous LT knockdown in MCC cells.
- Assessment of nuclear localization, RB-binding, and protein interactions (HSC-70, Vam6p).
- Evaluation of viral protein expression and induction of E2F target genes.
Main Results:
- Nuclear localization of MCPyV LT is essential for function, but a specific nuclear localization sequence is not required for proliferation.
- Large portions of the unique regions (MURs) and the ALTO protein are dispensable for MCPyV-driven MCC cell proliferation.
- MCPyV LT variants lacking entire MURs retain growth-promoting activity, though expression stability may be affected.
- HSC-70 interaction is significantly involved in MCPyV LT function, including growth promotion and E2F target gene induction, while Vam6p binding is not essential.
Conclusions:
- MCPyV LT's RB-binding domain and nuclear localization are critical for MCC growth.
- Specific viral unique regions and the ALTO protein are not essential for MCPyV-driven tumor cell proliferation.
- HSC-70 interaction plays a significant role in mediating MCPyV LT's oncogenic functions in MCC.
