Virus envelopes

Many viruses enclose their capsid in a membrane envelope, which they gain by the process of budding; the capsid wraps itself in a coating of lipid bilayer as it passes through the membrane. Many  virus  envelopes are the  result of budding from  the  plasma membrane (e.g. measles and  influenza viruses) but  some viruses bud  through intracellular membranes (e.g. herpes simplex virus, hepatitis C virus) such as the endoplasmic reticulum  (ER) or Golgi and  these enveloped virions are  subsequently transported out  of the cell within a vesicle,  hence they  temporarily have  a second membrane. Finally, some viruses (e.g. baculoviruses) gain an envelope without budding across any membrane, but the exact nature of this process is unknown. Indeed the process of budding itself is not fully understood, but it is known to involve specific domains within viral proteins that interact with cellular proteins, and also with the cellular ubiquitin-proteasome system. Virus envelopes can be loose and amorphous (e.g. herpes simplex virus) or tightly bound to the capsid (e.g. human immunodeficiency virus, HIV). Embedded within the virus envelope are a number of virus-encoded glycoproteins, which are involved in virus attachment to cells and serve as targets for the immune response. Most virion envelopes contain exclusively viral glycoproteins, but retroviruses (e.g. HIV) also contain host glycoproteins.