The Herpes Simplex Virus Tegument Protein pUL21 is Required for Viral Genome Retention Within Capsids

Abstract

During virion morphogenesis herpes simplex virus nucleocapsids transit from the nucleoplasm to the cytoplasm, through a process called nuclear egress, where the final stages of virion assembly occur. Coupled to nuclear egress is a poorly understood quality-control mechanism that preferentially selects genome-containing C-capsids, rather than A- and B-capsids that lack genomes, for transit to the cytoplasm. Cells infected with HSV strains deleted for the tegument protein pUL21 accumulate both empty A-capsids and C-capsids in the cytoplasm of infected cells. Quantitative microscopy experiments indicated that C-capsids were preferentially selected for envelopment at the inner nuclear membrane and that nuclear integrity remained intact in cells infected with pUL21 mutants, prompting alternative explanations for the accumulation of A-capsids in the cytoplasm. More A-capsids were also found in the nuclei of cells infected with pUL21 mutants suggesting pUL21 might be required for optimal genome packaging or genome retention within capsids. In support of this, viral genomes were prematurely released into the cytoplasm during pUL21 mutant infection that led to enhanced activation of cellular cytoplasmic DNA sensors. Analysis of WT and pUL21 mutant capsids revealed an increased association of the known pUL21 binding protein, pUL16, with pUL21 mutant capsids, suggesting that premature and/or enhanced association of pUL16 with capsids might result in capsid destabilization. Further supporting this idea, deletion of pUL16 from a pUL21 mutant strain rescued genome retention within capsids. These findings suggest that pUL21 regulates pUL16 addition to nuclear capsids and that premature, and/or, over-addition of pUL16 impairs HSV genome retention within capsids.

Herpes simplex virus 1 protein pUL21 alters ceramide metabolism by activating the interorganelle transport protein CERT

“…4 B ). Both pUL16 and pUL21 are known to facilitate efficient release of virus capsids from the nuclei of infected cells ( 74 ), and a recent study suggests that the pUL16–pUL21 interaction may help regulate capsid maturation and stability ( 75 ). Our mutagenesis is consistent with previous biochemical experiments that had mapped the binding of pUL16 to the C-terminal region of pUL21 (residues 268–535) and shown that the N-terminal 40 amino acids of pUL16 were dispensable for this interaction ( 10 ).…”