The ATP-driven chaperone valosin-containing protein (VCP)/p97 governs critical steps in ubiquitin-dependent protein quality control and intracellular signalling pathways. It cooperates with diverse partner proteins to help process ubiquitin-labelled proteins for recycling or degradation by the proteasome in many cellular contexts. Recent studies have uncovered unexpected cellular functions for p97 in autophagy, endosomal sorting and regulating protein degradation at the outer mitochondrial membrane, and elucidated a role for p97 in key chromatin-associated processes. These findings extend the functional relevance of p97 to lysosomal degradation and reveal a surprising dual role in protecting cells from protein stress and ensuring genome stability during proliferation.

The AAA-ATPase VCP/p97 cooperates with distinct cofactors to process ubiquitinated proteins in different cellular pathways 1–3. VCP missense mutations cause a systemic degenerative disease in humans, but the molecular pathogenesis is unclear 4, 5. We used an unbiased mass spectrometry approach and identified a VCP complex with the UBXD1 cofactor, which binds the plasma membrane protein caveolin-1 (Cav1) and whose formation is specifically disrupted by disease-associated mutations. We show that VCP-UBXD1 targets mono-ubiquitinated Cav1 in SDS-resistant high molecular weight complexes on endosomes, which are en route to degradation in endolysosomes 6. Expression of VCP mutant proteins, chemical inhibition of VCP, or siRNA-mediated depletion of UBXD1 leads to a block of Cav1 transport at the limiting membrane of enlarged endosomes in cultured cells. In patient muscle, muscle-specific Caveolin-3 (Cav3) accumulates in sarcoplasmic pools and specifically delocalises from the sarcolemma. These results extend the cellular functions of VCP to mediating sorting of ubiquitinated cargo in the endocytic pathway and suggest that impaired trafficking of caveolin may contribute to the pathogenesis in individuals with VCP mutations.

Rupture of endosomes and lysosomes is a major cellular stress condition leading to cell death and degeneration. Here, we identified an essential role for the ubiquitin-directed AAA-ATPase, p97, in the clearance of damaged lysosomes by autophagy. Upon damage, p97 translocates to lysosomes and there cooperates with a distinct set of cofactors including UBXD1, PLAA, and the deubiquitinating enzyme YOD1, which we term ELDR components for Endo-Lysosomal Damage Response. Together, they act downstream of K63-linked ubiquitination and p62 recruitment, and selectively remove K48-linked ubiquitin conjugates from a subpopulation of damaged lysosomes to promote autophagosome formation. Lysosomal clearance is also compromised in MEFs harboring a p97 mutation that causes inclusion body myopathy and neurodegeneration, and damaged lysosomes accumulate in affected patient tissue carrying the mutation. Moreover, we show that p97 helps clear late endosomes/lysosomes ruptured by endocytosed tau fibrils. Thus, our data reveal an important mechanism of how p97 maintains lysosomal homeostasis, and implicate the pathway as a modulator of degenerative diseases.

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Rationale Telethonin (also known as titin-cap or t-cap) is a 19 kDa Z-disk protein with a unique β-sheet structure, hypothesized to assemble in a palindromic way with the N-terminal portion of titin and to constitute a signalosome participating in the process of cardio-mechanosensing. In addition, a variety of telethonin mutations are associated with the development of several different diseases; however, little is known about the underlying molecular mechanisms and telethonin’s in vivo function. Objective Here we aim to investigate the role of telethonin in vivo and to identify molecular mechanisms underlying disease as a result of its mutation. Methods and Results By using a variety of different genetically altered animal models and biophysical experiments we show that, contrary to previous views, telethonin is not an indispensable component of the titin-anchoring system, nor is deletion of the gene or cardiac specific overexpression associated with a spontaneous cardiac phenotype. Rather, additional titin-anchorage sites, such as actin-titin crosslinks via α-actinin, are sufficient to maintain Z-disk stability despite the loss of telethonin. We demonstrate that a main novel function of telethonin is to modulate the turnover of the pro-apoptotic tumor suppressor p53 after biomechanical stress in the nuclear compartment, thus linking telethonin, a protein well known to be present at the Z-disk, directly to apoptosis (“mechanoptosis”). In addition, loss of telethonin mRNA and nuclear accumulation of this protein is associated with human heart failure, an effect which may contribute to enhanced rates of apoptosis found in these hearts. Conclusions Telethonin knockout mice do not reveal defective heart development or heart function under basal conditions, but develop heart failure following biomechanical stress, owing at least in part to apoptosis of cardiomyocytes, an effect which may also play a role in human heart failure.