The Globoside Receptor Triggers Structural Changes in the B19 Virus Capsid That Facilitate Virus Internalization

Abstract

Globoside (Gb4Cer), Ku80 autoantigen, and ␣5␤1 integrin have been identified as cell receptors/coreceptors for human parvovirus B19 (B19V), but their role and mechanism of interaction with the virus are largely unknown. In UT7/Epo cells, expression of Gb4Cer and CD49e (integrin alpha-5) was high, but expression of Ku80 was insignificant. B19V colocalized with Gb4Cer and, to a lesser extent, with CD49e. However, only anti-Gb4Cer antibodies could disturb virus attachment. Only a small proportion of cell-bound viruses were internalized, while the majority became detached from the receptor. When added to uninfected cells, the receptor-detached virus showed superior cell binding capacity and infectivity. Attachment of B19V to cells triggered conformational changes in the capsid leading to the accessibility of the N terminus of VP1 (VP1u) to antibodies, which was maintained in the receptor-detached virus. VP1u became similarly accessible to antibodies following incubation of B19V particles with increasing concentrations of purified Gb4Cer. The receptormediated exposure of VP1u is critical for virus internalization, since capsids lacking VP1 could bind to cells but were not internalized. Moreover, an antibody against the N terminus of VP1u disturbed virus internalization, but only when present during and not after virus attachment, indicating the involvement of this region in binding events required for internalization. These results suggest that Gb4Cer is not only the primary receptor for B19V attachment but also the mediator of capsid rearrangements required for subsequent interactions leading to virus internalization. The capacity of the virus to detach and reattach again would enhance the probability of productive infections.

Parvovirus B19 Uptake Is a Highly Selective Process Controlled by VP1u, a Novel Determinant of Viral Tropism

“…The clustering of several neutralizing epitopes in the VP1u region denotes the existence of motifs with essential functions in the virus life cycle. Although originally not accessible to antibodies (35), this protein domain becomes exposed following B19V binding to Gb4Cer receptor at the cell surface (30,31). The extracellular exposure of VP1u, which contrasts with the intracellular exposure observed in other parvoviruses (20,21,22), suggests a possible function in B19V internalization.…”

“…Similarly, colocalization of B19V with the ubiquitous CD29 (␤1 integrin subunit) or CD49e (␣5 integrin subunit) was not observed (unpublished data). UT7/Epo cells, which allow VP1u binding, virus internalization, and infection, do not express detectable levels of Ku80 (31). However, Ku80 may function as a primary receptor for B19V attachment in certain cells that do not express Gb4Cer (13).…”

“…The intracellular exposure of VP1u is thought to be important for endosomal escape (23,24,25,26,27) and nuclear targeting (22,28,29). In sharp contrast to other parvoviruses, VP1u of B19V becomes accessible to antibodies (Abs) upon binding to the Gb4Cer receptor (30,31). The specific role of the early exposure of VP1u at the cell surface has not been elucidated.…”

Characterization of the Early Steps of Human Parvovirus B19 Infection

“…In addition, nuclear localization signal (NLS) sequences have been identified in the N-VP1 proteins from some parvoviruses, which might assist in the transport of capsids toward the nucleus (57). We have recently shown that B19V is unique among parvoviruses in that N-VP1 becomes externalized upon receptor binding (7,9,45). Therefore, in contrast to other parvoviruses, B19V would not depend on low endosomal pH for this critical conformational rearrangement.…”

Parvovirus B19 Achievements and Challenges

“…This effect has been attributed to conformational changes in the capsid, triggered by attachment of B19V to cells and leading to the accessibility of the VP1u region. The receptor-mediated exposure of VP1u region is critical for virus internalization, since capsids lacking VP1 can bind to cells but are not internalized [76]. Then, Gb4Cer is not only the primary receptor for B19V attachment, but also the mediator of capsid rearrangements required for the interactions leading to virus internalization.…”