In this article we will discuss about the assembly, evolvement and release of herpesviruses.
Assembly of Herpesviruses:
There are more than 30 HSV-1 gene products that act as the structural components of the virion. These are expressed with late kinetics. Capsid assembly i.e. enveloping is a complex process.
The general outline of the process was established by careful electron microscopic studies by W. Gibson and co-workers at Johns Hopkins University. Homa and colleagues at the Research Lab of the Pharmacia and Upjohn Corporations provided a great impetus to the understanding of the biochemical steps in viral morphogenesis.
The surface virus protein [VPS (UL19)] and associated scaffolding proteins (UL26 and UL 26.5) of HSV capsids are synthesized in cytoplasm; from there, these are transported to the nucleus to form scaffold. The pro-capsid proteins (UL18, UL19 and UL38) assemble around the proteins (UL26 and UL26.5) in the nucleus; thereafter, these are digested.
The empty capsid incorporates DNA by the action of cleavage/packaging proteins. Then the other viral proteins interact with replicated viral DNA to allow DNA encapsidation. The highly basic polyamines synthesized with viral enzymes facilitate the encapsidation process. The tegument (matrix) proteins presumably associate near the nuclear membrane to form full capsids (Fig. 17.15).
Envelopment and Release of Herpesviruses:
Fig. 17.16 shows envelopment and release of virus. The envelopment of alpha herpesviruses is a complex process. The capsid migrates to the nuclear membrane and buds into the lumen between the inner and outer nuclear membrane.
The inner nuclear membrane consists of viral glycoproteins. Viral glycoproteins are translated from HSV RNA on the rough endoplasmic reticulum then transported to the Golgi body in vesicles to continue the glycosylation process. The glycoproteins are then transported in vesicles to the nuclear or plasma membrane.
In the early maturation process in the nucleus, capsids is surrounded by the primary tegument protein (UL31) which directs budding through the inner nuclear membrane into which the phosphorylated membrane protein (UL31 and UL34) has been inserted. These primarily enveloped capsids then bud off through the outer nuclear membrane where the primary envelope is lost.
Then the cytoplasmic capsids associate with the numerous tegument proteins of the mature virion, including α-TIF and vhs, which help in final envelopment. The membranes of exocytotic vesicles contain all the glycoproteins associated with the mature virions.
Final envelopment takes place as the mature capsids and associated tegument proteins bud off into the exocytotic vesicles. Infectious virions can be released from the cell in exocytotic vesicles: They may also remain associated within these vesicles and spread to uninfected cells through virus-induced fusion. 