SARS-CoV-2 Protein Modifies Calcium Signals via SERCA - Study

The E protein, a structural protein of the SARS-CoV-2 virus, has been found to co-localize with SERCA2b and its regulators in the endoplasmic reticulum (ER), according to recent research. The study focused on how this viral protein interacts with these components of the ER's calcium signaling system. 
 
SERCA2b is an integral membrane protein that resides within the ER and is responsible for transporting calcium ions from the cytosol into the lumen of this organelle. This process is crucial for maintaining cellular homeostasis and regulating various physiological processes such as muscle contraction, neurotransmission, cell growth, apoptosis, etc. Regulins are proteins that regulate SERCA activity by binding directly to it. 
 
In order to investigate whether E protein affects SERCA function or not, researchers expressed both proteins together in HeLa cells along with certain receptors, including phospholamban (PLN), aryl hydrocarbon receptor nuclear translocator-like (ALN), and emerin (ELN). They observed that E-protein primarily localized in ER, where it co-localized significantly with PLN, ALN, and ELN, as well as with SERCA2b. 
 
However, mere localization doesn't necessarily indicate interaction between two entities, so they used the Förster Resonance Energy Transfer (FRET) technique, which showed high FRET values indicating strong interaction between E-proteins and members of SERCAs/regulin systems. Furthermore, through molecular dynamics simulations, they discovered overlapping binding sites between PLNs/E proteins and Sercas, suggesting potential competition between them for accessing Serca-binding sites. 
 
Interestingly enough, when they conducted experiments aimed at assessing the functional significance behind this interaction involving changes in ER Ca levels upon ATP administration, the results obtained were quite intriguing: in the presence of E-protein, a rapid decrease was noticed, hinting towards sensitization caused by it leading towards calcium depletion from the storehouse, i.e., the endoplasmic reticulum. 
 
In addition, co-immunoprecipitation experiments confirmed the interaction between SERCA and the E protein, providing further evidence of their association. This discovery could be significant as it may provide a new avenue for understanding how SARS-CoV-2 affects cellular functions and potentially opens up new possibilities for therapeutic interventions. 
 
The researchers also performed computational biology studies to model the structures of these complexes. Although they were not able to accurately predict structures involving SERCA and regulins due to the transient nature of the interactions involved, they did succeed in generating some plausible models using PIPER/ClusPro software, which showed docked E-proteins and regulins at the same site where PLN was observed in the experimental structure, suggesting potential competition amongst them for accessing the Serca-binding site. 
 
Furthermore, molecular dynamic simulations revealed that although both PLN and E-proteins exhibited overlapping binding sites with Serca, the latter's interactions were less frequent, hinting towards lesser stability compared to the former. 
 
Finally, through monitoring ER Ca levels under different conditions, such as the presence or absence of ATP or mCherry-tagged E protein, investigators concluded that viral protein doesn't affect passive Ca leakage from ER but does interfere with its uptake by slowing down rate, significantly hinting towards the inhibitory effect it might have on SERCA function. 
 
Overall, this study provides crucial insights into how SARS-CoV-2's structural protein interacts with the host cell’s calcium signaling machinery, thereby affecting its normal functioning and leading towards possible pathogenesis caused by the virus upon infection.