BRIEF REPORT Description of a novel single mutation in the AcMNPV polyhedrin gene that results in abnormally large cubic polyhedra Marı ´a Gabriela Lo ´pez • Victoria Alfonso • Elisa Carrillo • Oscar Taboga Received: 18 August 2010 / Accepted: 4 December 2010 / Published online: 19 December 2010 Ó Springer-Verlag 2010 Abstract We describe a point mutation in the AcMNPV polyhedrin gene that produces abnormally large cubic polyhedra in packaging cell lines. A polyhedrin mutant baculovirus in which the single change E44G was intro- duced confirmed that this mutation and no other alterations in the AcMNPV genome was responsible for the abnormal phenotype. Although baculoviral VP39 protein was detec- ted inside mutant polyhedra, electron microscopy demon- strated that only a proportion of the large crystals allow occlusion of virions. When compared with wild-type polyhedra, the mutant inoculum showed reduced oral infectivity for Rachiplusia nu larvae. Hence, the amino acid 44 substitution in the AcMNPV polyhedrin protein alters polyhedrin assembly and affects viral occlusion and infectivity. Keywords AcMNPV polyhedrin Á Mutant Á Large polyhedra Virions of a number of invertebrate viruses are specifically occluded within robust crystalline particles. The occlusion bodies (OBs) act as protective packages, allowing infec- tious virions to survive for long periods in harsh environ- ments, protecting them from UV irradiation and desiccation. This unique mode of propagation within pro- tein crystals is found only in two unrelated families of invertebrate viruses: Baculoviridae (baculovirus) and Reoviridae (cypovirus). This characteristic may have evolved to maintain the virus during periods when the larval population is too small, for example during diapause or seasonal hostile conditions [1, 2]. Baculoviruses of the genus Alphabaculovirus have several virus particles occluded in a polyhedrin crystalline matrix to form OBs or polyhedra. Feeding larvae become infected by ingesting OBs that dissolve in the alkaline environment of the midgut, releasing the occlusion-derived virus (ODV). Polyhedrin is one of the most conserved proteins of the virus. It has a molecular mass of around 29 kDa, and it is highly expressed in the late steps of infection in insect cells. How polyhedrin plays its role in the occlusion process is not yet well understood. It is believed that formation of OBs depends on interactions between polyhedrin and other proteins present on the envelope of the virion [3, 4], but some studies have dem- onstrated that it also depends on its own amino acid sequence [5, 6]. Three spontaneous mutant viruses with altered polyhedron morphology have been isolated and characterized in Autographa californica multiple nucleo- polyhedrovirus (AcMNPV). In each case, a single amino acid change in the polyhedrin sequence was responsible for the abnormal phenotype of the OBs. One of those mutants, bearing a substitution at position 118, produced large amounts of small particles instead of crystalline OBs [7]. The other reported mutants, which had a substitution at residue 59 [8] or 25 [9], produced a single cubic polyhe- dron in the nucleus of the infected cell. These polyhedron- deficient mutants have been classified by their different characteristics as class i and class ii, respectively [10]. This characterization is based on the location of point mutations in the polyhedrin secondary structure. According to these authors, class i mutations affect b sheet regions, in the M. G. Lo ´pez (&) Á E. Carrillo Á O. Taboga Instituto de Biotecnologı ´a, CICVyA, INTA-Castelar Cc25, B1712WAA Buenos Aires, Argentina e-mail: glopez@cnia.inta.gov.ar V. Alfonso Á E. Carrillo Á O. Taboga CONICET, 1917 Rivadavia, C.A.B.A., Argentina 123 Arch Virol (2011) 156:695–699 DOI 10.1007/s00705-010-0885-8