Incorporation of cellular proteins into enveloped virus particles
Sammanfattning: This thesis work aimed to investigate the assembly and budding of enveloped virus particles with focus on the fate of cellular proteins, present in or near the plasma membrane (PM) where the budding occurs. It was previously shown that compact viruses, like alphaviruses, with a covering outer protein coat, did not contain any cellular proteins in the envelope. However, cellular proteins were found in purified retroviral preparations and these proteins were thought to be specifically incorporated into the viral envelopes. We asked whether cellular proteins instead were randomly incorporated during viral budding at the PM and if selection of proteins occurred at all. Hence, we built up systems to metabolically label lipids and proteins in cells both before and during infection and production of virus particles, to isolate and purify PMs and released virus particles and to equalize the preparations to each other thus enabling direct comparable analyses of protein contents and amounts of proteins in the two samples. In this thesis we can, for the chosen model retroviruses Moloney murine leukaemia virus (Mo-MuLV) and Human immunodeficiency virus type-1 (HIV-1), show that the bulk of the cellular proteins, present in or near the budding sites, are in a non-selective manner incorporated into assembling and budding particles. Only a few proteins were excluded from or concentrated into the purified particles, as compared to isolated PMs, which was opposite to the general idea. Some viruses, including Human herpesvirus 6A (HHV-6A), have been associated with the autoimmune disease multiple sclerosis (MS). One theory for induction of the autoimmune reactions is that incorporated cellular proteins are presented to immune system cells together with viral peptides, leading to misdirected immune reactions. We asked whether MV-6A incorporates cellular proteins during assembly and release of viral particles. Indeed, we could conclude that purified, intact and infectious HHV-6A particles contained host proteins, among them we identified clathrin, ezrin, actin, Tsg101 and the CD46 protein which is also used as a receptor for the virus. Altogether, our results suggest that the majority of the proteins, present at or near the assembly and budding sites of retroviral particles, are incorporated into the particles in a nonselective manner. The work also suggests that enveloped viruses belonging to other viral families incorporate cellular proteins and this opens up for questions regarding, for example, induction of autoimmune diseases.
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