Functional integrity of the SEL1L-HRD1 complex is critical for ERAD and organismal viability.

The SEL1L-HRD1 complex represents the most evolutionarily conserved branch of endoplasmic reticulum-associated degradation (ERAD), with SEL1L acting as a key cofactor for the E3 ubiquitin ligase HRD1. While the physiological relevance of this complex has been increasingly recognized, whether SEL1L is strictly required for HRD1 function in mammals has remained unclear. Here, using complementary in vivo and in vitro approaches, we define the architecture and physiological significance of the mammalian SEL1L-HRD1 ERAD complex. Our data demonstrate that direct binding between SEL1L and HRD1 is essential for ERAD function and neonatal survival in mice. In three knock-in mouse models harboring targeted mutations at the SEL1L-HRD1 interface, we show that the L709P variant - unlike the benign P699T mutation - results in complete neonatal lethality within 30 hours of birth, a phenotype more severe than that of the partially lethal S658P variant. Mechanistically, the L709P mutation abolishes SEL1L-HRD1 interaction, disrupting substrate engagement and impairing recruitment of the E2 enzyme UBE2J1, leading to the accumulation and aggregation of misfolded proteins in the ER. Notably, these defects can be partially rescued by HRD1 overexpression, echoing findings from yeast. Together, our results provide definitive evidence that the SEL1L-HRD1 interaction is essential for ERAD activity and neonatal viability in mammals, resolving a long-standing question in ERAD biology and identifying a new therapeutic strategy for modulating ERAD activity in humans.