Membrane cholesterol regulates the oligomerization and fusogenicity of SARS-CoV fusion peptide: implications in viral entry.

N-terminal residues (770-788) of the S2 glycoprotein of severe acute respiratory syndrome coronavirus (SARS-CoV) have been recognized as a potential fusion peptide that can be involved in the entry of the virus into the host cell. Membrane composition plays an important role in lipid-peptide interaction and the oligomeric status of the peptide. SARS-CoV fusion peptide (S2 fusion peptide) is known to undergo cholesterol-dependent oligomerization in the membrane; however, its significance in membrane fusion is still speculative. This study aimed to investigate the oligomerization of SARS-CoV fusion peptide in a membrane containing phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol, with varying concentrations of cholesterol, and to evaluate peptide-induced membrane fusion to correlate the importance of peptide oligomerization with membrane fusion. Peptide-induced modulation of membrane organization and dynamics was explored by steady-state and time-resolved fluorescence spectroscopic measurements using depth-dependent probes. The results clearly demonstrated the induction of S2 fusion peptide oligomerization by membrane cholesterol and the higher efficiency of the oligomer in promoting membrane fusion compared to its monomeric counterpart. Cholesterol-dependent peptide oligomerization and membrane fusion are important aspects of viral infection since the cholesterol level can change with age as well as with the onset of various pathophysiological conditions.

Mechanism of Membrane Fusion: Interplay of Lipid and Peptide.

Membrane fusion is an essential process for the survival of eukaryotes and the entry of enveloped viruses into host cells. A proper understanding of the mechanism of membrane fusion would provide us a handle to manipulate several biological pathways, and design efficient vaccines against emerging and re-emerging viral infections. Although fusion proteins take the central stage in catalyzing the process, role of lipid composition is also of paramount importance. Lipid composition modulates membrane organization and dynamics and impacts the lipid-protein (peptide) interaction. Moreover, the intrinsic curvature of lipids has strong impact on the formation of stalk and hemifusion diaphragm. Detection of transiently stable intermediates remains the bottleneck in the understanding of fusion mechanism. In order to circumvent this challenge, analytical methods can be employed to determine the kinetic parameters from ensemble average measurements of observables, such as lipid mixing, content mixing, and content leakage. The current review aims to present an analytical method that would aid our understanding of the fusion mechanism, provides a better insight into the role of lipid shape, and discusses the interplay of lipid and peptide in membrane fusion.

Enhanced Cholesterol-Dependent Hemifusion by Internal Fusion Peptide 1 of SARS Coronavirus-2 Compared to Its N-Terminal Counterpart.

Membrane fusion is an important step for the entry of the lipid-sheathed viruses into the host cells. The fusion process is being carried out by fusion proteins present in the viral envelope. The class I virus contains a 20-25 amino acid sequence at its N-terminal of the fusion domain, which is instrumental in fusion and is called as a "fusion peptide". However, severe acute respiratory syndrome (SARS) coronaviruses contain more than one fusion peptide sequences. We have shown that the internal fusion peptide 1 (IFP1) of SARS-CoV-2 is far more efficient than its N-terminal counterpart (FP) to induce hemifusion between small unilamellar vesicles. Moreover, the ability of IFP1 to induce hemifusion formation increases dramatically with growing cholesterol content in the membrane. Interestingly, IFP1 is capable of inducing hemifusion but fails to open the pore.

Membrane cholesterol and SARS-CoV-2 infection: a possible connection

The article describes a probable relationship between membrane cholesterol and viral infection with special attention to coronavirus infection. It discusses the challenges in monitoring the cholesterol level of infected population. It mentions that monitoring blood cholesterol level of the infected population might provide better insight into the relationship between cholesterol and morbidity.

Entry Inhibitors: Efficient Means to Block Viral Infection.

The emerging and re-emerging viral infections are constant threats to human health and wellbeing. Several strategies have been explored to develop vaccines against these viral diseases. The main effort in the journey of development of vaccines is to neutralize the fusion protein using antibodies. However, significant efforts have been made in discovering peptides and small molecules that inhibit the fusion between virus and host cell, thereby inhibiting the entry of viruses. This class of inhibitors is called entry inhibitors, and they are extremely efficient in reducing viral infection as the entry of the virus is considered as the first step of infection. Nevertheless, these inhibitors are highly selective for a particular virus as antibody-based vaccines. The recent COVID-19 pandemic lets us ponder to shift our attention towards broad-spectrum antiviral agents from the so-called 'one bug-one drug' approach. This review discusses peptide and small molecule-based entry inhibitors against class I, II, and III viruses and sheds light on broad-spectrum antiviral agents.

Mechanistic insights of host cell fusion of SARS-CoV-1 and SARS-CoV-2 from atomic resolution structure and membrane dynamics.

The emerging and re-emerging viral diseases are continuous threats to the wellbeing of human life. Previous outbreaks of Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS had evidenced potential threats of coronaviruses in human health. The recent pandemic due to SARS-CoV-2 is overwhelming and has been going beyond control. Vaccines and antiviral drugs are ungently required to mitigate the pandemic. Therefore, it is important to comprehend the mechanistic details of viral infection process. The fusion between host cell and virus being the first step of infection, understanding the fusion mechanism could provide crucial information to intervene the infection process. Interestingly, all enveloped viruses contain fusion protein on their envelope that acts as fusion machine. For coronaviruses, the spike or S glycoprotein mediates successful infection through receptor binding and cell fusion. The cell fusion process requires merging of virus and host cell membranes, and that is essentially performed by the S2 domain of the S glycoprotein. In this review, we have discussed cell fusion mechanism of SARS-CoV-1 from available atomic resolution structures and membrane binding of fusion peptides. We have further discussed about the cell fusion of SARS-CoV-2 in the context of present pandemic situation.