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Loetzerich, Mark (2007): Analysis of the nuclear egress complex of mouse cytomegalovirus. Dissertation, LMU München: Faculty of Biology
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Abstract

Herpesvirus infections are usually asymptomatic or associated with mild symptoms. Fatal diseases are seen in immune suppressed and immune incompetent individuals. Although herpesviruses are of emerging medical importance nowadays an infection only can be controlled by chemotherapeutics, which target viral DNA replication and cause negative side effects. Essential steps of herpesvirus morphogenesis might indicate new targets for interference. The export of the 110 nm large herpesvirus nucleocapsid from the cell nucleus is a logistic problem, because such a cargo exceeds the size tolerated by the nuclear pore complex. Two conserved herpes simplex virus 1 proteins, UL31 and UL34, form a complex at the inner nuclear membrane and govern nuclear egress of herpesvirus nucleocapsids. In mouse cytomegalovirus (MCMV), a member of the beta-herpesvirus subfamily, the homologous proteins M53/p38 and M50/p35 form the nuclear egress complex (NEC). The interaction of these proteins is essential for the virus and might serve as a potential drug target. Here we describe a saturating random mutagenesis procedure for the UL31 homologue M53/p38. From a total of 498 individual mutants 72 were reinserted into the genome to test virus complementation. The analysis revealed that the N- terminus of M53/p38 provides the nuclear localization signal (NLS). The M53/p38 binding site for the NEC partner M50/p35 was located to aa 112-137. No single aa exchange for alanine could destroy NEC formation but virus attenuation revealed a major role for the aa K128, Y129, and L130. Further, the lethal phenotype of several insertion and stop mutants indicated the functional importance of the C-terminus of the protein, which might serve to construct dominant negative mutants. The interference of herpesvirus proteins with unknown cellular functions is of emerging interest. MCMV nucleocapsid formation is followed by a complex process of nucleocapsid transitions through cellular membrane barriers. The re-organization of the nuclear architecture by viral proteins probably involves the interaction with host cell proteins. Here, we found that the NEC of MCMV interacts with an important inner nuclear membrane protein complex, the lamin B receptor, which controls nuclear membrane stability. Thus, herpesviruses might target major cellular principles that govern nuclear integrity.