The endoplasmic reticulum (ER) and mitochondria form contacts that support communication between these two organelles, including synthesis and transfer of lipids, and the exchange of calcium, which regulates ER chaperones, mitochondrial ATP production, and apoptosis. Despite the fundamental roles for ER–mitochondria contacts, little is known about the molecules that regulate them. Here we report the identification of a multifunctional sorting protein, PACS‐2, that integrates ER–mitochondria communication, ER homeostasis, and apoptosis. PACS‐2 controls the apposition of mitochondria with the ER, as depletion of PACS‐2 causes BAP31‐dependent mitochondria fragmentation and uncoupling from the ER. PACS‐2 also controls formation of ER lipid‐synthesizing centers found on mitochondria‐associated membranes and ER homeostasis. However, in response to apoptotic inducers, PACS‐2 translocates Bid to mitochondria, which initiates a sequence of events including the formation of mitochondrial truncated Bid, the release of cytochrome c , and the activation of caspase‐3, thereby causing cell death. Together, our results identify PACS‐2 as a novel sorting protein that links the ER–mitochondria axis to ER homeostasis and the control of cell fate, and provide new insights into Bid action.

Herpes simplex virus 1 (HSV-1) is an important human pathogen and a paradigm for virus-induced host shut-off. Here we show that global changes in transcription and RNA processing and their impact on translation can be analysed in a single experimental setting by applying 4sU-tagging of newly transcribed RNA and ribosome profiling to lytic HSV-1 infection. Unexpectedly, we find that HSV-1 triggers the disruption of transcription termination of cellular, but not viral, genes. This results in extensive transcription for tens of thousands of nucleotides beyond poly(A) sites and into downstream genes, leading to novel intergenic splicing between exons of neighbouring cellular genes. As a consequence, hundreds of cellular genes seem to be transcriptionally induced but are not translated. In contrast to previous reports, we show that HSV-1 does not inhibit co-transcriptional splicing. Our approach thus substantially advances our understanding of HSV-1 biology and establishes HSV-1 as a model system for studying transcription termination.

Alphaherpesviruses like herpes simplex virus are large DNA viruses characterized by their ability to establish lifelong latent infection in neurons. As for all herpesviruses, alphaherpesvirus virions contain a protein-rich layer called “tegument” that links the DNA-containing capsid to the glycoprotein-studded membrane envelope. Tegument proteins mediate a diverse range of functions during the virus lifecycle, including modulation of the host-cell environment immediately after entry, transport of virus capsids to the nucleus during infection, and wrapping of cytoplasmic capsids with membranes (secondary envelopment) during virion assembly. Eleven tegument proteins that are conserved across alphaherpesviruses have been implicated in the formation of the tegument layer or in secondary envelopment. Tegument is assembled via a dense network of interactions between tegument proteins, with the redundancy of these interactions making it challenging to determine the precise function of any specific tegument protein. However, recent studies have made great headway in defining the interactions between tegument proteins, conserved across alphaherpesviruses, which facilitate tegument assembly and secondary envelopment. We summarize these recent advances and review what remains to be learned about the molecular interactions required to assemble mature alphaherpesvirus virions following the release of capsids from infected cell nuclei.

PACS‐1 is a cytosolic protein involved in controlling the correct subcellular localization of integral membrane proteins that contain acidic cluster sorting motifs, such as furin and human immunodeficiency virus type 1 (HIV‐1) Nef. We have now investigated the interaction of PACS‐1 with heterotetrameric adaptor complexes. PACS‐1 associates with both AP‐1 and AP‐3, but not AP‐2, and forms a ternary complex between furin and AP‐1. A short sequence within PACS‐1 that is essential for binding to AP‐1 has been identified. Mutation of this motif yielded a dominant‐negative PACS‐1 molecule that can still bind to acidic cluster motifs on cargo proteins but not to adaptor complexes. Expression of dominant‐negative PACS‐1 causes a mislocalization of both furin and mannose 6‐phosphate receptor from the trans‐Golgi network, but has no effect on the localization of proteins that do not contain acidic cluster sorting motifs. Furthermore, expression of dominant‐negative PACS‐1 inhibits the ability of HIV‐1 Nef to downregulate MHC‐I. These studies demonstrate the requirement for PACS‐1 interactions with adaptor proteins in multiple processes, including secretory granule biogenesis and HIV‐1 pathogenesis.

The assembly and egress of herpesviruses are complex processes that require the budding of viral nucleocapsids into the lumen of cytoplasmic compartments to form mature infectious virus. This envelopment stage shares many characteristics with the formation of luminal vesicles in multivesicular endosomes. Through expression of dominant-negative Vps4, an enzyme that is essential for the formation of luminal vesicles in multivesicular endosomes, we now show that Vps4 function is required for the cytoplasmic envelopment of herpes simplex virus type 1. This is the first example of a large enveloped DNA virus engaging the multivesicular endosome sorting machinery to enable infectious virus production.