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J. Biol. Chem., Vol. 279, Issue 40, 41573-41579, October 1, 2004

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Conformational Changes of Murine Polyomavirus Capsid Proteins Induced by Sialic Acid Binding^*

Michaela Cavaldesi, Maddalena Caruso, Olga Sthandier, Paolo Amati, and Marie Isabelle Garcia¶

From the Istituto Pasteur-Fondazione Cenci Bolognetti, Dipartimento di Biotecnologie Cellulari ed Ematologia, Sezione di Genetica Molecolare, Università di Roma "La Sapienza," Viale Regina Elena 324, 00161 Rome, Italy

Murine polyomavirus (Py) infection initiates by the recognition of cell membrane molecules containing terminal sialic acid (SA) residues through specific binding pockets formed at the major capsid protein VP1 surface. VP1 Pockets 1, 2, and 3 bind terminal SA, Gal, and second branched SA, respectively. The consequence of recognition on viral cell entry remains elusive. In this work, we show that preincubation of Py with soluble compounds within Pocket 1 (N-acetyl or N-glycolyl neuraminic acids) increases Py cell binding and infectivity in murine 3T6 fibroblasts. In contrast, Gal does not significantly alter Py binding nor infectivity, whereas sialyllactose, in Pockets 1 and 2, decreases cell binding and infectivity. Binding experiments with Py virus-like particles confirmed the direct involvement of VP1 in this effect. To determine whether such results could reflect VP1 conformational changes induced by SA binding, protease digestion assays were performed after pretreatment of Py or virus-like particles with soluble receptor fragments. Binding of SA with the VP1 Pocket 1, but not of compounds interacting with Pocket 2, was associated with a transition of this protein from a protease-sensitive to a protease-resistant state. This effect was transmitted to the minor capsid proteins VP2 and VP3 in virus particles. Attachment of Py to cell monolayers similarly led to a VP1 trypsin-resistant pattern. Taken together, these data present evidence that initial binding of Py to terminal SA induces conformational changes in the viral capsid, which may influence subsequent virus cell entry steps.

Received for publication, May 28, 2004 , and in revised form, July 13, 2004.

^* This work has been supported by grants (Fondo per gli Investimenti della Ricerca di Base and Ateneo) from the Ministry of University and Research of Italy and by funds from the Italian Spatial Agency. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Funded by a fellowship from the Adriano Buzzati-Traverso Foundation.

To whom correspondence may be addressed. Tel.: 39-06490393; Fax: 39-064462891; E-mail: amati{at}bce.uniroma1.it. ¶ To whom correspondence may be addressed. Tel.: 39-06490393; Fax: 39-064462891; E-mail: garcia{at}bce.uniroma1.it.

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