The Oncogenic Small Tumor Antigen of Merkel Cell Polyomavirus Is an Iron-Sulfur Cluster Protein That Enhances Viral DNA Replication

Abstract

Merkel cell polyomavirus (MCPyV) plays an important role in Merkel cell carcinoma (MCC).A ccumulating evidence has suggested a role for Merkel cell polyomavirus (MCPyV) in the development of a lethal skin cancer, Merkel cell carcinoma (MCC), making it the first polyomavirus to be conclusively associated with human cancer (1). MCC tumors develop rapidly and are highly metastatic. It is one of the most aggressive skin cancers with a high mortality rate of 33% (which exceeds the rate of melanoma) (2), and a 5-year observed survival rate of less than 45% (3). High seroprevalence for MCPyV in the adult human population and analyses of healthy human skin suggest that MCPyV is a common component of the normal skin flora (4, 5).MCPyV has a circular, double-stranded DNA genome of ϳ5 kb (6). A regulatory region (RR) separates the early and late regions of the viral genome (6). The RR contains the viral origin of replication (Ori) and bidirectional promoters for viral transcription. The early region encodes large T (LT) and small T (sT) antigens, the 57kT antigen, and a recently discovered protein called alternative LT open reading frame (ORF) (ALTO) (6, 7). The late region encodes the capsid proteins, VP1 and VP2 (8,9). It is well established that clonal integration of the MCPyV genome into the host genome is a key event in the development of MCPyV-associated MCC tumors (10). Integration or other mutagenic events almost invariably result in truncation of LT upstream of its C-terminal helicase domain, rendering the mutant protein defective for mediating viral replication (10). Although both LT and sT antigens are often required for MCPyV-positive MCC cell survival and proliferation (11,12), sT has emerged as the key oncogenic driver in MCC carcinogenesis. This is supported by the observation that sT expression can transform rodent fibroblasts, whereas the expression of LT, or truncated LT found in MCC tumors, cannot (12). MCPyV sT also demonstrates robust transforming activity in transgenic mouse model systems (13).Due to differential splicing, LT and sT share an N-terminal domain with homology to cellular DnaJ chaperone proteins. In sT, the DnaJ motif is followed by an sT-unique C-terminal do- Citation Tsang SH, Wang R, Nakamaru-Ogiso E, Knight SAB, Buck CB, You J. 2016. The oncogenic small tumor antigen of Merkel cell polyomavirus is an iron-sulfur cluster protein that enhances viral DNA replication. J Virol 90:1544 -1556.

Protein-mediated viral latency is a novel mechanism for Merkel cell polyomavirus persistence

“…MCV sT is generated from alternative splicing of the T antigen encoding gene to form an iron-sulfur protein with an unclear function for the virus (20,42). Similar to other polyomavirus sT proteins, MCV sT binds and inactivates some PP2A isoforms (43); however, it is transforming in rodent cells (44) and induces cell proliferation and tumorigenicity in transgenic mice (45)(46)(47).…”

Merkel cell polyomavirus and Merkel cell carcinoma

“…The ST unique region contains two zinc fingers that may be used to bind iron. When it was produced in bacteria, MCPyV ST coordinated iron sulfur [2Fe-2S] and [4Fe-4S] [110]. Whether MCPyV ST binds Fe or Zn in mammalian cells is not clear.…”

“…The LSD motif in ST appears to increase the levels of MCPyV LT and contributes to increased viral replication at least in part by increasing LT levels [111]. Evidence for a role of MCPyV ST in MCPyV replication includes its ability to translocate to viral DNA replication centres within the nucleus in the presence of the viral origin and LT [110]. The LT stabilizing activity may reflect ST's ability to perturb the function of FBXW7, a component of the cullin RING ligase family of ubiquitin ligases.…”

Analysis of NFU‐1 metallocofactor binding‐site substitutions—impacts on iron–sulfur cluster coordination and protein structure and function

“…For NFU1, the altered (CXCXXC) motif in the derivative protein resembles other cluster-binding sequences that have been previously identified. The same sequence occurs twice in a small T antigen from Merkel cell polyomavirus that is capable of binding a [2Fe-2S] cluster at one site, and a [4Fe-4S] cluster with a fourth ligand of uncertain identity at another [15]. Similarly, a cytosolic Fe/S cluster assembly protein, anamorsin (Dre2 in yeast) utilizes a CXXCXC sequence to bind a [2Fe-2S] cluster that resembles the motif found in the NFU1 disease state [16].…”