Phosphorylation-dependent changes in the spatial relationship between Ca-ATPase polypeptide chains in sarcoplasmic...

Area of Science:

• Biochemistry
• Molecular Biology
• Membrane Protein Dynamics

Background:

• The Ca-ATPase enzyme in sarcoplasmic reticulum membranes plays a crucial role in calcium ion transport.
• Understanding the structural dynamics of Ca-ATPase coupling mechanisms is essential for elucidating its function.
• Previous studies have suggested potential conformational changes during the enzyme's catalytic cycle.

Purpose of the Study:

• To investigate structural changes within and between Ca-ATPase molecules during its functional cycle.
• To differentiate between intramolecular and intermolecular structural dynamics using fluorescence resonance energy transfer (FRET).
• To determine if calcium binding or phosphoenzyme formation induces significant conformational alterations.

Main Methods:

• Utilized fluorescence resonance energy transfer (FRET) between covalently bound spectroscopic probes (IAEDANS, fluorescein, DABmal).
• Employed time-correlated single photon counting to measure energy transfer efficiencies.
• Differentiated intra- and intermolecular FRET by comparing native membranes with detergent-solubilized or reconstituted ATPase chains.

Main Results:

• No significant changes in intramolecular FRET were observed upon calcium binding or phosphoenzyme formation.
• Intermolecular FRET between IAEDANS and DABmal/Fmal increased upon phosphoenzyme intermediate formation.
• This increase in intermolecular FRET was reversible upon calcium addition and specific to certain probe pairs.

Conclusions:

• The study found no evidence of large structural changes within the B tryptic fragment or between the A1 and B fragments of Ca-ATPase.
• Formation of the phosphoenzyme intermediate leads to a reorientation of associated Ca-ATPase polypeptide chains.
• These findings provide insights into the intermolecular communication and conformational dynamics of Ca-ATPase during its catalytic cycle.