Structural basis for the high Ca2+ affinity of the ubiquitous SERCA2b Ca2+ pump

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Physiology

Background:

• Sarco(endo)plasmic reticulum Ca(2+) ATPase (SERCA) transporters regulate intracellular Ca(2+) gradients essential for cell functions.
• SERCA activity is modulated by Ca(2+) ion affinity, crucial for physiological demand.
• The SERCA2b isoform exhibits high Ca(2+) affinity due to a unique C-terminal extension (2b-tail).

Purpose of the Study:

• To elucidate the structure-function relationship of the SERCA2b C-terminal extension (2b-tail) in controlling Ca(2+) affinity.
• To understand the molecular mechanism by which the 2b-tail enhances SERCA2b's Ca(2+) binding.
• To explore the therapeutic potential of targeting the 2b-tail interaction site.

Main Methods:

• Extensive structure-function analysis using SERCA2b mutants and SERCA1a2b chimeras.
• Nuclear Magnetic Resonance (NMR) structural determination of the TM11 segment.
• Mutagenesis studies guided by structural data.

Main Results:

• The 2b-tail, comprising a transmembrane segment (TM11) and luminal extension, functionally cooperates with other SERCA domains (TM7/TM10, luminal loops).
• This interaction stabilizes the Ca(2+)-bound E1 conformation, altering transport kinetics and increasing apparent Ca(2+) affinity.
• A structural model for SERCA2b was developed, revealing a mechanism similar to Na(+),K(+)-ATPase subunit interactions.

Conclusions:

• The 2b-tail is a key determinant of SERCA2b's high Ca(2+) affinity through specific structural interactions.
• The identified 2b-tail mechanism provides a molecular basis for SERCA2b's enhanced Ca(2+) transport.
• Targeting the 2b-tail interaction site could be a novel strategy to improve SERCA2a function in cardiac diseases.