Lassa virus is a notorious human pathogen that infects many thousands of people each year in West Africa, causing severe viral hemorrhagic fevers and significant mortality. The surface glycoprotein of Lassa virus mediates receptor recognition through its GP1 subunit. Here we report the crystal structure of GP1 from Lassa virus, which is the first representative GP1 structure for Old World arenaviruses. We identify a unique triad of histidines that forms a binding site for LAMP1, a known lysosomal protein recently discovered to be a critical receptor for internalized Lassa virus at acidic pH. We demonstrate that mutation of this histidine triad, which is highly conserved among Old World arenaviruses, impairs LAMP1 recognition. Our biochemical and structural data further suggest that GP1 from Lassa virus may undergo irreversible conformational changes that could serve as an immunological decoy mechanism. Together with a variable region that we identify on the surface of GP1, those could be two distinct mechanisms that Lassa virus utilizes to avoid antibody-based immune response. IMPORTANCEStructural data at atomic resolution for viral proteins is key for understanding their function at the molecular level and can facilitate novel avenues for combating viral infections. Here we used X-ray protein crystallography to decipher the crystal structure of the receptor-binding domain (GP1) from Lassa virus. This is a pathogenic virus that causes significant illness and mortality in West Africa. This structure reveals the overall architecture of GP1 domains from the group of viruses known as the Old World arenaviruses. Using this structural information, we elucidated the mechanisms for pH switch and binding of Lassa virus to LAMP1, a recently identified host receptor that is critical for successful infection. Lastly, our structural analysis suggests two novel immune evasion mechanisms that Lassa virus may utilize to escape antibody-based immune response. L assa Virus (LASV) belongs to the Arenaviridae family of enveloped, negative-stranded RNA viruses (1). Arenaviruses are zoonotic viruses that are carried and spread to humans by rodents (2). Infection by some members of this family leads to severe viral hemorrhagic fevers (VHF) (2). LASV is the most predominant of the viruses causing VHF, with an estimated 300,000 annual cases in western Africa and high mortality rates (3). Arenaviruses are subdivided into two major subgroups, the "Old World" (OW) and the "New World" (NW) arenaviruses, which are endemic to Africa and South America, respectively (4).Arenaviruses utilize various cell surface proteins as their cellular receptors for recognizing and attaching to target cells. NW arenaviruses that belong to clades A and B use transferrin receptor 1 (TfR1) (5, 6), whereas OW arenaviruses, as well as clade C NW arenaviruses, use ␣-dystroglycan (␣-DG) (7-9). A trimeric class 1 viral glycoprotein complex (the spike complex) recognizes the cellular receptors and mediates membrane fusion upon exposure to low pH at the lysosom...

To effectively infect cells, Lassa virus needs to switch in an endosomal compartment from its primary receptor, ␣-dystroglycan, to a protein termed LAMP1. A unique histidine triad on the surface of the receptor-binding domain from the glycoprotein spike complex of Lassa virus is important for LAMP1 binding. Here we investigate mutated spikes that have an impaired ability to interact with LAMP1 and show that although LAMP1 is important for efficient infectivity, it is not required for spike-mediated membrane fusion per se. Our studies reveal important regulatory roles for histidines from the triad in sensing acidic pH and preventing premature spike triggering. We further show that LAMP1 requires a positively charged His230 residue to engage with the spike complex and that LAMP1 binding promotes membrane fusion. These results elucidate the molecular role of LAMP1 binding during Lassa virus cell entry and provide new insights into how pH is sensed by the spike. IMPORTANCELassa virus is a devastating disease-causing agent in West Africa, with a significant yearly death toll and severe long-term complications associated with its infection in survivors. In recent years, we learned that Lassa virus needs to switch receptors in a pHdependent manner to efficiently infect cells, but neither the molecular mechanisms that allow switching nor the actual effects of switching were known. Here we investigate the activity of the viral spike complex after abrogation of its ability to switch receptors. These studies inform us about the role of switching receptors and provide new insights into how the spike senses acidic pH.

Obesity is a global epidemic causing morbidity and impaired quality of life. The melanocortin receptor 4 (MC4R) is at the crux of appetite, energy homeostasis, and body-weight control in the central nervous system and is a prime target for anti-obesity drugs. Here, we present the cryo-EM structure of the human MC4R-Gs signaling complex bound to the agonist setmelanotide, a cyclic peptide recently approved for the treatment of obesity. The work reveals the mechanism of MC4R activation, highlighting a molecular switch that initiates satiation signaling. In addition, our findings indicate that Ca2+ is required for agonist but not antagonist efficacy. These results fill a gap in understanding MC4R activation and could guide the design of future weight management drugs.

Cell entry of many enveloped viruses occurs by engagement with cellular receptors, followed by internalization into endocytic compartments and pH-induced membrane fusion. A previously unnoticed step of receptor switching was found to be critical during cell entry of two devastating human pathogens: Ebola and Lassa viruses. Our recent studies revealed the functional role of receptor switching to LAMP1 for triggering membrane fusion by Lassa virus and showed the involvement of conserved histidines in this switching, suggesting that other viruses from this family may also switch to LAMP1. However, when we investigated viruses that are genetically close to Lassa virus, we discovered that they cannot bind LAMP1. A crystal structure of the receptor-binding module from Morogoro virus revealed structural differences that allowed mapping of the LAMP1 binding site to a unique set of Lassa residues not shared by other viruses in its family, illustrating a key difference in the cell-entry mechanism of Lassa virus that may contribute to its pathogenicity.

Bariatric surgery dramatically improves glycemic control, yet the underlying molecular mechanisms remain controversial because of confounding weight loss. We performed sleeve gastrectomy (SG) on obese and diabetic leptin receptor-deficient mice (/). One week postsurgery, mice weighed 5% less and displayed improved glycemia compared with sham-operated controls, and islets from SG mice displayed reduced expression of diabetes markers. One month postsurgery SG mice weighed more than preoperatively but remained near-euglycemic and displayed reduced hepatic lipid droplets. Pair feeding of SG and sham / mice showed that surgery rather than weight loss was responsible for reduced glycemia after SG. Although insulin secretion profiles from islets of sham and SG mice were indistinguishable, clamp studies revealed that SG causes a dramatic improvement in muscle and hepatic insulin sensitivity accompanied by hepatic regulation of hepatocyte nuclear factor-α and peroxisome proliferator-activated receptor-α targets. We conclude that long-term weight loss after SG requires leptin signaling. Nevertheless, SG elicits a remarkable improvement in glycemia through insulin sensitization independent of reduced feeding and weight loss.