The spike (S) protein of SARS-CoV-2 mediates receptor binding and cell entry and is the dominant target of the immune system. S exhibits substantial conformational flexibility. It transitions from closed to open conformations to expose its receptor binding site, and subsequently from prefusion to postfusion conformations to mediate fusion of viral and cellular membranes. S protein derivatives are components of vaccine candidates and diagnostic assays, as well as tools for research into the biology and immunology of SARS-CoV-2. Here we have designed mutations in S which allow production of thermostable, crosslinked, S protein trimers that are trapped in the closed, pre-fusion, state. We have determined the structures of crosslinked and non-crosslinked proteins, identifying two distinct closed conformations of the S trimer. We demonstrate that the designed, thermostable, closed S trimer can be used in serological assays. This protein has potential applications as a reagent for serology, virology and as an immunogen.

Rapid COVID-19 diagnosis in the hospital is essential, although this is complicated by 30%–50% of nose/throat swabs being negative by SARS-CoV-2 nucleic acid amplification testing (NAAT). Furthermore, the D614G spike mutant dominates the pandemic and it is unclear how serological tests designed to detect anti-spike antibodies perform against this variant. We assess the diagnostic accuracy of combined rapid antibody point of care (POC) and nucleic acid assays for suspected COVID-19 disease due to either wild-type or the D614G spike mutant SARS-CoV-2. The overall detection rate for COVID-19 is 79.2% (95% CI 57.8–92.9) by rapid NAAT alone. The combined point of care antibody test and rapid NAAT is not affected by D614G and results in very high sensitivity for COVID-19 diagnosis with very high specificity.

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21HMG-CoA reductase (HMGCR), the rate-limiting enzyme of the cholesterol biosynthetic 22 pathway and the therapeutic target of statins, is post-transcriptionally regulated by sterol-23 accelerated degradation. Under cholesterol-replete conditions, HMGCR is ubiquitinated and 24 degraded, but the identity of the E3 ubiquitin ligase(s) responsible for mammalian HMGCR 25 turnover remains controversial. Using systematic, unbiased CRISPR/Cas9 genome-wide 26 screens with a sterol-sensitive endogenous HMGCR reporter, we comprehensively map the 27 E3 ligase landscape required for sterol-accelerated HMGCR degradation. We find that 28 RNF145 and gp78, independently co-ordinate HMGCR ubiquitination and degradation. 29 RNF145, a sterol-responsive ER-resident E3 ligase, is unstable but accumulates following 30 sterol depletion. Sterol addition triggers RNF145 recruitment to HMGCR and Insig-1, 31promoting HMGCR ubiquitination and proteasome-mediated degradation. In the absence of 32 both RNF145 and gp78, Hrd1, a third UBE2G2-dependent ligase partially regulates HMGCR 33 activity. Our findings reveal a critical role for the sterol-responsive RNF145 in HMGCR 34 regulation and elucidate the complexity of sterol-accelerated HMGCR degradation. 35

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