Role of HSV alpha TIF in Acid Blob Activation

Role of HSV coded VP16 protein in Acid blob activation:


Introduction:
HSV type I & II are the members of class Beta virus of herpesviridae family. Which generally causes facial genital herpatic lessons. Members belongs to this class are neurotrophic in nature. Herpes Simplex virus bears nearly 120 -150 genes, which involved the structural as well regulatory genes. One among such protein VP16 plays a dual role in the HSV life cycle. In other words VP16 itself a structural protein, which is very necessary for stability of virion partical and also acts as a vital regulatory factor in order to initiate the transcription of Immediate early genes.


VP16 protein in Acid blob activation or in transcription activation:
One of the HSV type I coded structural protein, VP16 also referred as UL 48, α – TIF or tegument protein acts transiently in order to initiate the transcription of immediate early genes. This is one of the cases where a structural protein plays a role of functional one in the Virion.

VP16 along with the some other cellular binding proteins like Oct I and CTF binds co-operatively to enhancer element present upstream to the α-promoter. Enhancer element has the sequence TATAGARAT, where R represents any of the nucleotide base. So any variation at this site doesn’t affect the binding affinity of VP16,as VP16 has broad specificity against the sequences of enhancer element. Co-operative binding at the enhancer sequence induce transient initiation of transcription. The above mentioned proteins along with VP16 are transacting ones.

Co-operative binding of VP16-Oct1-CTF mobilize the RNA polymerase, which located on the promoter in order to transcribe α – immediate early genes of HSV.The principle mechanism behind this is the nature and kind of amino acids present at the C – terminal of VP16.
Structural analysis of VP16 revealed that, there are more number of acidic amino acids present at the C – terminal of VP16.These amino acids effectively result in the mobility of RNA polymerase from an α promoter. This phenomenon is generally known as “Acid blob activation”. (Due to the acidic nature of VP16 in order to acts as trans inducing factors (α – TIF).

VP16 mediated acid blob activation is considered to be a good example for those viral proteins, which themselves important for providing structural stability to the virus but also involved in the processes like regulation of viral gene expression.