Structure and function of the C -terminal region of a fusion receptor for herpes simplex virus.

Abstract

Herpes simplex virus (HSV) enters most human cells through pH-independent membrane fusion that requires four viral glycoproteins, a receptor for gD, and other unknown cellular molecules. We previously isolated uncharacterized human cDNAs, <italic>hfl-A27</italic> and <italic>hfl-B5</italic>, in a functional screen for HSV cellular receptors. I have focused on function and structure of the C-terminal region of the <italic>hfl-B5</italic> product that is important for HSV infection, and on initial analysis of <italic>hfl-A27</italic>. <italic>Hfl-B5</italic> expresses a new class of HSV receptor, B5, that contains an alpha-helical heptad repeat (HR) domain at its C-terminus that functions in HSV entry and infection. The alpha-helix region alone as a 30mer synthetic peptide was able to mediate membrane fusion. Proteins from sequences with point mutations in this region that are predicted to abolish the alpha-helix structure did not cause fusion. The 30mer peptide previously shown to block HSV infection, induced fusion of human cells or artificial liposomes in the absence of any viral molecules while altered B5 peptides did not. Structural analyses showed that the B5 peptide formed an alpha-helix and interacted with lipids. The B5 receptor is the first cellular protein shown to function directly in membrane fusion for HSV. This function in viral-cell or cell-cell fusion occurs in part through the C-terminal alpha-helix that can form coiled coils similar to those of class I fusion proteins. The product of <italic>hfl-A27</italic>, A27, is a 37 kDa membrane-associated protein that allows HSV strain-dependent entry. Recent reports indicate that A27 is part of the newly characterized human CNOT1, a 2376 as global regulator of transcription. It is likely that the entire cDNA of CNOT1 was not contained in the <italic>hfl-A27</italic> plasmid used in the functional screen. That only a portion of the CNOT1 protein was sufficient for HSV entry attests to the versatility of HSV to utilize numerous human molecules or complexes to initiate infection. These results lead to a compelling model for HSV entry through protein interactions regulated in part through coiled coil receptor-ligand interactions. Further, they define fusion molecules as potential target sites for developing HSV therapeutics.