Everything about Hhv Capsid Portal Protein totally explained
HHV Capsid Portal Protein, or
HSV-1 UL-6 protein, is the
protein which forms a cylindrical portal in the
capsid of
Herpes simplex virus (HSV-1). The protein is commonly referred to as the
HSV-1 UL-6 protein because it's is the
transcription product of Herpes
gene U
L-6.
The Herpes viral
DNA enters and exits the capsid via the capsid portal. The capsid portal is formed by twelve copies of portal protein arranged as a ring; the proteins contain a
leucine zipper sequence of
amino acids which allow them to adhere to each other. Each
icosahedral capsid contains a single portal, located in one vertex.
The portal is formed during initial capsid assembly and interacts with
scaffolding proteins that construct the procapsid.
When the capsid is nearly complete, the viral DNA enters the capsid (i.e, the DNA is
encapsidated) by a mechanism invovling the portal and a DNA-binding protein complex similar to
bacteriophage terminase. Multiple studies suggest an
evolutionary relationship between Capsid Portal Protein and bacteriophage portal proteins.
The genetic sequence of HSV-1 gene U
L-6 is conserved across the Herpesviridae family and this family of genes is known as the "Herpesvirus UL6-like" gene family. "U
L-6" is
nomenclature meaning that the protein is
genetically encoded by the sixth (6th)
open reading frame found in the viral genome segment named "Unique-Long (U
L)".
Studies
| Studies by amino acid sequence location |
| pUL-6 Amino acid range |
Summary |
Reference |
| E121, A618, Q621 |
Point mutations confer resistance to portal assembly inhibitor WAY-150138 |
van Zeijl, et al., 2000 |
| 198-295 |
Deletion mutant forms immature B-capsids with no portals |
Nellissery, et al., 2007 |
| pUL-26.5 "Scaffolding protein" Amino acid range |
Summary |
Reference |
| 143-151 |
Deletion inhibits UL-6 portal assembly |
Singer, et al., 2005 |
Dodecameric structure
Research performed in 2004 used electron microscopy to predict that U
L-6 forms 11, 12, 13, and 14-unit polymers. The dodecameric (twelve-unit) form was found to be most likely.
Refinements to the electron microscopy in 2007 allowed finding that the portal is a twelve (12)-unit polymer present at one of the twelve capsid vertices instead of the U
L-19 pentamer found at non-portal vertices.
Leucine zipper creates inter-protein adhesion
A study using deletion and mutation of the U
L-6 amino acid sequence demonstrated the leucine residues in a predicuted leucine zipper motif were required for formation of the dodecameric ring sturcutre.
Early involvement in capsid assembly
Assembly of portal units is an initial step in constructing capsids of viral progeny. Capsids assembled in the absence of portals lack portals.
Interaction with capsid scaffolding protein
In 2003, gel eletrophoresis studies demonstrated that intact U
L-6 portals associate
in vitro with viral protein U
L-26. This association is antagonized by that action of WAY-150138, a
thiourea inhibitor of HHV encapsidation.
Further investigation during 2006 showed that assembly of capsid with portal depends on interaction of U
L-6 with "scaffolding" protein U
L-26.5, amino acids 143 through 151.
Interaction with terminase complex
U
L-6 associates with a U
L-15/U
L-28 protein complex during capsid assembly. The U
L-15/U
L-28 is believed to bind with viral DNA and serve the same purpose as
terminase by packing viral DNA into the capsid during capsid assembly.
Function during DNA egress
The DNA exits the capsid in a single linear segment. DNA exit may be controlled by U
L-6 and dependent on temperature or environmental proteins.
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