👤 Nadav Elad

Proper protein targeting to organelles is crucial for maintaining eukaryotic cellular function and homeostasis. This necessity has driven the evolution of specific targeting signals on proteins and the targeting factors that recognize them. A prominent example is peroxisomal matrix proteins, most of which depend on the targeting factor Pex5 to localize and function correctly. Although most Pex5 cargoes contain a peroxisomal targeting signal type 1 (PTS1), they are not all targeted similarly. Some undergo priority targeting, facilitated either by stronger binding to specific subsets of PTS1 signals or by additional interaction interfaces. These observations highlight the extensive complexity of Pex5-mediated targeting. In this study, we reveal that the Saccharomyces cerevisiae (yeast) matrix protein Eci1 can reach peroxisomes and bind Pex5 in the absence of PTS1. By solving the structure of the yeast Pex5-Eci1 complex using cryo-electron microscopy, we identified additional binding interfaces. Our findings provide new insights into the versatile interactions between Pex5 and its cargo, Eci1. More broadly, this work highlights the intricate, dynamic nature of the interactions between cargo factors and their cargoes to meet the complex environment within eukaryotic cells. Show less

SARS-CoV-2 variants of interest and concern will continue to emerge for the duration of the COVID-19 pandemic. To map mutations in the receptor-binding domain (RBD) of the spike protein that affect binding to angiotensin-converting enzyme 2 (ACE2), the receptor for SARS-CoV-2, we applied in vitro evolution to affinity-mature the RBD. Multiple rounds of random mutagenic libraries of the RBD were sorted against decreasing concentrations of ACE2, resulting in the selection of higher affinity RBD binders. We found that mutations present in more transmissible viruses (S477N, E484K and N501Y) were preferentially selected in our high-throughput screen. Evolved RBD mutants include prominently the amino acid substitutions found in the RBDs of B.1.620, B.1.1.7 (Alpha), B1.351 (Beta) and P.1 (Gamma) variants. Moreover, the incidence of RBD mutations in the population as presented in the GISAID database (April 2021) is positively correlated with increased binding affinity to ACE2. Further in vitro evolution increased binding by 1,000-fold and identified mutations that may be more infectious if they evolve in the circulating viral population, for example, Q498R is epistatic to N501Y. We show that our high-affinity variant RBD-62 can be used as a drug to inhibit infection with SARS-CoV-2 and variants Alpha, Beta and Gamma in vitro. In a model of SARS-CoV-2 challenge in hamster, RBD-62 significantly reduced clinical disease when administered before or after infection. A 2.9 Å cryo-electron microscopy structure of the high-affinity complex of RBD-62 and ACE2, including all rapidly spreading mutations, provides a structural basis for future drug and vaccine development and for in silico evaluation of known antibodies. Show less