Members of the polyomavirus (PyV) family, including simian virus 40 (SV40), murine PyV (mPyV), and BK virus (BKV), bind cell surface gangliosides to initiate infection (2
). PyV capsids are assembled from 72 pentamers (capsomeres) of the major coat protein VP1, with the internal proteins VP2 and VP3 buried within the capsids (7
). The VP1 pentamer makes direct contact with the carbohydrate portion of the ganglioside (10
) and dictates the specificity of virus interaction with the cell. Gangliosides are glycolipids that contain a ceramide domain inserted into the plasma membrane and a carbohydrate domain that directly binds the virus. Specifically, SV40 binds to ganglioside GM1 (2
), mPyV binds to gangliosides GD1a and GT1b (11
), and BKV binds to gangliosides GD1b and GT1b (8
Recently, a new human PyV designated Merkel cell PyV (MCPyV) was identified in Merkel cell carcinomas, a rare but aggressive skin cancer of neuroendocrine origin (3
). It is as yet unclear whether MCPyV is the causative agent of Merkel cell carcinomas (17
). A key to understanding the infectious and transforming properties of MCPyV is the elucidation of its cellular entry pathway. In this study, we identify a putative host cell receptor for MCPyV.
Because an intact infectious MCPyV has not yet been isolated, we generated recombinant MCPyV VP1 pentamers in order to characterize cellular factors that bind to MCPyV. VP1 capsomeres have been previously shown to be equivalent to virus with respect to hemagglutination properties (4
), and the atomic structure of VP1 bound to sialyllactose has demonstrated that the capsomere is sufficient for this interaction (12
). The MCPyV VP1 protein (strain w162) was expressed and purified as described previously (1
). Briefly, a glutathione S
-transferase-MCPyV VP1 fusion protein was expressed in Escherichia coli
and purified using glutathione-Sepharose affinity chromatography. The fusion protein was eluted using glutathione and cleaved in solution with thrombin. The thrombin-cleaved sample was then rechromatographed on a second glutathione-Sepharose column to remove glutathione transferase and any uncleaved protein. The unbound VP1 was then chromatographed on a P-11 phosphocellulose column, and peak fractions eluting between 400 and 450 mM NaCl were collected. The purified protein was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), followed by Coomassie blue staining (Fig. , left) and immunoblotting using an antibody (I58) that generally recognizes PyV VP1 proteins (Fig. , right) (9
). Transmission electron microscopy (Philips CM10) analysis confirmed that the purified recombinant MCPyV VP1 formed pentamers (capsomeres), which did not assemble further into virus-like particles (Fig. ). In an initial screening of its cell binding properties, we tested whether the MCPyV VP1 pentamers hemagglutinated red blood cells (RBCs). The MCPyV VP1 pentamers were incubated with sheep RBCs and assayed as previously described (5
). SV40 and mPyV recombinant VP1 pentamers served as negative and positive controls, respectively. We found that MCPyV VP1 hemagglutinated the RBCs with the same efficiency as mPyV VP1 (protein concentration/hemagglutination unit) (Fig. , compare rows B and C from wells 1 to 11), suggesting that MCPyV VP1 engages a plasma membrane receptor on the RBCs. The recombinant murine VP1 protein used for comparison was from the RA strain, a small plaque virus (4
). Thus, MCPyV VP1 has the hemagglutination characteristics of a small plaque mPyV (12
FIG. 1. Characterization of MCPyV VP1. Recombinant MCPyV VP1 forms pentamers and hemagglutinates sheep RBCs. (A) Coomassie blue-stained SDS-PAGE and an immunoblot of the purified recombinant MCPyV VP1 protein are shown. Molecular mass markers are indicated. (B) (more ...)
To characterize the chemical nature of the putative receptor for MCPyV, total membranes from RBCs were purified as described previously (15
). The plasma membranes (30 μg) were incubated with MCPyV VP1 (0.5 μg) and floated on a discontinuous sucrose gradient (15
). After fractionation, the samples were analyzed by SDS-PAGE, followed by immunoblotting with I58. VP1 was found in the bottom of the gradient in the absence of the plasma membranes (Fig. , first panel). In the presence of plasma membranes, a fraction of the VP1 floated to the middle of the gradient (Fig. , second panel), supporting the hemagglutination results that suggested that MCPyV VP1 binds to a receptor on the plasma membrane.
FIG. 2. MCPyV VP1 binds to a protease-resistant, sialic acid-containing receptor on the plasma membrane. (A) Purified recombinant MCPyV VP1 was incubated with or without the indicated plasma membranes. The samples were floated in a discontinuous sucrose gradient, (more ...)
To determine whether the receptor is a protein or a lipid, plasma membrane preparations (30 μg) were incubated with proteinase K (Sigma), followed by analysis with SDS-PAGE and Coomassie blue staining. Under these conditions, the majority of the proteins in the plasma membranes were degraded by the protease (Fig. , compare lanes 1 and 2). Despite the lack of proteins, the proteinase K-treated plasma membranes bound MCPyV VP1 as efficiently as control plasma membranes (Fig. , compare the second and third panels), demonstrating that MCPyV VP1 interacts with a protease-resistant receptor. The absence of VP1 in the bottom fraction in Fig. (third panel) is consistent with the fact that the buoyant density of the membranes is lowered by proteolysis. Of note, a similar result was seen with binding of the mPyV to the plasma membrane (15
). Binding of MCPyV to the cell surface of two human tissue culture cells (i.e., HeLa and 293T) was also largely unaffected by pretreatment of the cells with proteinase K (Fig. , compare lanes 1 and 2), further indicating that a nonproteinaceous molecule on the plasma membrane engages the virus.
We next determined whether the protease-resistant receptor contains a sialic acid modification. Plasma membranes (10 μg) were incubated with a neuraminidase (α2-3,6,8 neuraminidase; Calbiochem) to remove the sialic acid groups. In contrast to the control plasma membranes, the neuraminidase-treated membranes did not bind MCPyV VP1 (Fig. , compare first and second panels), indicating that the MCPyV receptor includes a sialic acid modification.
Gangliosides are lipids that contain sialic acid modifications. We asked if MCPyV VP1 binds to gangliosides similar to other PyV family members. The structures of the gangliosides used in this analysis (gangliosides GM1, GD1a, GD1b, and GT1b) are depicted in Fig. . To assess a possible ganglioside-VP1 interaction, we employed a liposome flotation assay established previously (15
). When liposomes (consisting of phosphatidyl-choline [19 μl of 10 mg/ml], -ethanolamine [5 μl of 10 mg/ml], -serine [1 μl of 10 mg/ml], and -inositol [3 μl of 10 mg/ml]) were incubated with MCPyV VP1 and subjected to the sucrose flotation assay, the VP1 remained in the bottom fraction (Fig. , first panel), indicating that VP1 does not interact with these phospholipids. However, when liposomes containing GT1b (1 μl of 1 mM), but not GM1 (1 μl of 1 mM) or GD1a (1 μl of 1 mM), were incubated with MCPyV VP1, the vesicles bound this VP1 (Fig. ). A low level of virus floated partially when incubated with liposomes containing GD1b (Fig. ), perhaps reflecting a weak affinity between MCPyV and GD1b. Importantly, MCPyV binds less efficiently to neuraminidase-treated GT1b-containing liposomes than to GT1b-containing liposomes (Fig. , sixth panel), suggesting that the GT1b sialic acids are involved in virus binding. This finding is consistent with the ability of neuraminidase to block MCPyV binding to the plasma membrane (Fig. ). The level of virus flotation observed in the neuraminidase-treated GT1b-containing liposomes is likely due to the inefficiency of the neuraminidase reaction with a high concentration of GT1b used to prepare the vesicles.
FIG. 3. MCPyV VP1 binds to ganglioside GT1b. (A) Structures of gangliosides GM1, GD1a, GD1b, and GT1b. The nature of the glycosidic linkages is indicated. (B) Purified MCPyV VP1 protein was incubated with liposomes only or with liposomes containing the indicated (more ...)
As controls, GM1-containing liposomes bound SV40 (Fig. ), GD1a-containing liposomes bound mPyV (Fig. ), and GD1b-containing liposomes bound BKV (Fig. ), demonstrating that the liposomes were functionally intact. We note that, while all of the MCPyV VP1 floated when incubated with liposomes containing GT1b (Fig. , sixth panel), a significant fraction of SV40, mPyV, and BKV VP1 remained in the bottom fraction despite being incubated with liposomes containing their respective ganglioside receptors (Fig. , second panels). This result is likely due to the fact that in contrast to MCPyV, which are assembled as pentamers (Fig. ), the SV40, mPyV, and BKV used in these experiments are fully assembled particles: their larger and denser nature prevents efficient flotation. Nonetheless, we conclude that MCPyV VP1 binds to ganglioside GT1b efficiently.
The observation that GD1a does not bind to MCPyV VP1 suggests that the monosialic acid modification on the right branch of GT1b (Fig. ) is insufficient for binding. Similarly, the failure of GD1b to bind MCPyV VP1 suggests that the sialic acid on the left arm of GT1b is necessary for binding. Together, these observations suggest that MCPyV VP1 interacts with sialic acids on both branches of GT1b (Fig. ). A recent structure of SV40 VP1 in complex with the sugar portion of GM1 (10
) demonstrated that although SV40 VP1 binds both branches of GM1 (Fig. ), only a single sialic acid in GM1 is involved in this interaction. In the case of mPyV, structures of mPyV VP1 in complex with different carbohydrates (12
) revealed that the sialic acid-galactose moiety on the left branch of GD1a (and GT1b) is sufficient for mPyV VP1 binding (Fig. ). Although no structure of BKV in complex with the sugar portion of GD1b (or GT1b) is available, in vitro binding studies (8
) have suggested that the disialic acid modification on the right branch of GD1b (and GT1b) is responsible for binding BKV VP1 (Fig. ). Thus, it appears that the unique feature of the MCPyV VP1-GT1b interaction is that the sialic acids on both branches of this ganglioside are likely involved in capsid binding.
A potential model of the different VP1-ganglioside interactions (see the text for discussion).
The identification of a potential cellular receptor for MCPyV will facilitate the study of its entry mechanism. An important issue for further study is to determine whether MCPyV targets Merkel cells preferentially, and if so, whether GT1b is found in higher levels in these cells to increase susceptibility.