Downloaded from www.microbiologyresearch.org by IP: 54.162.190.106 On: Mon, 08 Feb 2016 00:36:03 Remodelling of the nuclear lamina during human cytomegalovirus infection: role of the viral proteins pUL50 and pUL53 Daria Camozzi, 1 Sara Pignatelli, 1 Cecilia Valvo, 1 Giovanna Lattanzi, 2 Cristina Capanni, 2 Paola Dal Monte 1 and Maria Paola Landini 1 Correspondence Daria Camozzi daria.camozzi@unibo.it 1 Department of Clinical and Experimental Medicine, Division of Microbiology, University of Bologna, St Orsola General Hospital, Via Massarenti 9, 40138 Bologna, Italy 2 IGM-CNR, Unit of Bologna, c/o IOR, Via di Barbiano 1/10, 40136 Bologna, Italy Received 14 August 2007 Accepted 16 November 2007 A fundamental step in the efficient production of human cytomegalovirus (HCMV) progeny is viral egress from the nucleus to the cytoplasm of infected cells. In the family Herpesviridae, this process involves alteration of nuclear lamina components by two highly conserved proteins, whose homologues in HCMV are named pUL50 and pUL53. This study showed that HCMV infection induced the mislocalization of nuclear lamins and that pUL50 and pUL53 play a role in this event. At late stages of infection, both lamin A/C and lamin B showed an irregular distribution on the nuclear rim, coincident with areas of pUL53 accumulation. No variations in the total amount of nuclear lamins could be detected, supporting the view that HCMV induces a qualitative, rather than a quantitative, alteration of these cellular components, as has been suggested previously for other herpesviruses. Interestingly, pUL53, in the absence of other viral products, localized diffusely in the nucleus, whilst the co-expression and interaction of pUL53 with its partner, pUL50, restored its nuclear rim localization in distinct patches, thus indicating that pUL50 is sufficient to induce the localization of pUL53 observed during virus infection. Importantly, analysis of the nuclear lamina in the presence of pUL50–pUL53 complexes at the nuclear boundary and in the absence of other viral products showed that the two viral proteins were sufficient to promote alterations of lamins, strongly resembling those observed during HCMV infection. These results suggest that pUL50 and pUL53 may play an important role in the exit of virions from the nucleus by inducing structural modifications of the nuclear lamina. INTRODUCTION Whilst herpesvirus genome packaging and capsid forma- tion occur in the nuclear compartment, all subsequent steps of maturation take place in the cytoplasm, the final site of viral particle assembly. The most widely accepted model for herpesvirus nuclear egress suggests that capsids leave the nucleus through budding events at the nuclear envelope (Mettenleiter, 2004; Mettenleiter et al., 2006; Severi et al., 1988; Skepper et al., 2001; Stackpole, 1969), which consists of two leaflets, the inner and the outer nuclear membrane (INM and ONM, respectively) sepa- rated by the perinuclear space; nuclear capsids acquire a temporary envelope at the INM, which is subsequently lost by fusion with the ONM, allowing access to the cytosol. However, before crossing the nuclear envelope, virions have to overcome a massive obstacle underlying the INM, represented by the nuclear lamina, a thick meshwork of proteins associating in a highly organized structure. The main components of the nuclear lamina are the lamins, which, based on their expression patterns, properties and location, can be divided into two main classes: A-type lamins, including lamin A, AD10, C and C2; and B-type lamins, including lamin B1, B2 and B3 (Broers et al., 1997, 2006; Rzepecki, 2002; Stuurman et al., 1998). Several studies have shown that herpesvirus infection causes structural and biochemical rearrangements of nuclear lamina components, to ensure the efficient production of viral progeny (Leach et al., 2007; Marschall et al., 2005; Morris et al., 2007; Mou et al., 2007; Muranyi et al., 2002; Radsak et al., 1989; Reynolds et al., 2004; Scott & O’Hare, 2001). Notably, murine cytomegalovirus (MCMV) and, more recently, herpes simplex virus type 1 (HSV-1) have been reported to recruit cellular protein kinases C (PKCs) at the nuclear rim, and, as a consequence, induce an increase in phosphorylation of nuclear envelope proteins, including lamins and emerin, to promote nuclear lamina dissolution (Leach et al., 2007; Morris et al., 2007; Muranyi et al., 2002; Park & Baines, 2006). In addition, in HCMV, the virally encoded kinase pUL97 has been reported to be involved in this process by mediating the hyperpho- sphorylation of lamin A/C and p32, a component of the Journal of General Virology (2008), 89, 731–740 DOI 10.1099/vir.0.83377-0 0008-3377 G 2008 SGM Printed in Great Britain 731