Human cytomegalovirus protein pUL36: a dual cell death pathway inhibitor

Abstract

Human cytomegalovirus (HCMV) is an important human pathogen and a paradigm of intrinsic, innate and adaptive viral immune evasion. Here, we employed multiplexed tandem mass tag-based proteomics to characterise host proteins targeted for degradation late during HCMV infection. This approach revealed that mixed lineage kinase domain-like protein (MLKL), a key terminal mediator of cellular necroptosis, was rapidly and persistently degraded by the minimally passaged HCMV strain Merlin but not the extensively passaged strain AD169. The strain Merlin viral inhibitor of apoptosis pUL36 was necessary and sufficient both to degrade MLKL and to inhibit necroptosis. Furthermore, mutation of pUL36 Cys 131 abrogated MLKL degradation and restored necroptosis. As the same residue is also required for pUL36-mediated inhibition of apoptosis by preventing proteolytic activation of pro-caspase 8, we define pUL36 as a multifunctional inhibitor of both apoptotic and necroptotic cell death. SIGNIFICANCE STATEMENTCell death is a key defence against viral infection, preventing spread from infected to uninfected cells.Correspondingly, certain viruses encode inhibitors of apoptotic and necroptotic cell death pathways in order to facilitate their persistence. Human cytomegalovirus (HCMV) is an important human pathogen that can block apoptosis, but hitherto it has been unclear whether or how the virus blocks necroptosis.Here, we used a proteomic screen to identify human proteins targeted for destruction by HCMV, finding that the key necroptosis mediator MLKL is degraded throughout infection. MLKL is targeted for degradation by HCMV protein pUL36, which is also instrumental in inhibiting apoptosis. Thus, pUL36 is a dual cell death pathway inhibitor, and may represent an important therapeutic target.

Human cytomegalovirus protein pUL36: A dual cell death pathway inhibitor

“…The mass spectrometry proteomics data have been deposited to the ProteomeXchange Consortium ( www.proteomexchange.org/ ) via the PRIDE partner repository ( 56 ) with the dataset identifier PXD017279 ( 57 ). All materials described in this manuscript, and any further details of protocols employed, can be obtained on request from the corresponding author by email to mpw1001@cam.ac.uk .…”

Location, location, location: A compartmentalized view of TNF-induced necroptotic signaling

“…Necroptosis is a caspase-independent form of programmed cell death (1). Although necroptosis likely arose as a host response to counter pathogens that block apoptosis (2)(3)(4)(5)(6)(7)(8)(9), dysregulated necroptosis also plays a role in the pathologies of numerous noncommunicable diseases, including inflammatory diseases (10)(11)(12), inflammatory bowel disease (13), and kidney ischemia-reperfusion injury (14,15). Accordingly, there is widespread interest in the pharmacological inhibition of necroptotic cell death, with several pathway inhibitors having progressed into early phase clinical trials [as reviewed in (16)].…”

The regulation of necroptosis by post-translational modifications

“…In keeping with its proinflammatory nature, necroptosis has likely evolved as an innate immunity mechanism to complement apoptosis as a means of eliminating pathogens, such as in scenarios where pathogen-encoded proteins block apoptosis [11,12]. The idea that necroptosis arose as an ancestral host defense mechanism is supported by the discovery of viral and bacterial proteins that block necroptosis at various steps of the signaling pathway in animals [13][14][15][16][17][18][19][20][21][22][23], and the recent discovery of convergently-evolved pathway effectors that contribute to plant host defense [24].…”