Differential modulation of SERCA2 isoforms by calreticulin
L M John1, J D Lechleiter, P Camacho
1Department of Biomedical Engineering, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908, USA.
Abstract:
In Xenopus laevis oocytes, overexpression of calreticulin suppresses inositol 1,4,5-trisphosphate-induced Ca2+ oscillations in a manner consistent with inhibition of Ca2+ uptake into the endoplasmic reticulum. Here we report that the alternatively spliced isoforms of the sarcoendoplasmic reticulum Ca2+-ATPase (SERCA)2 gene display differential Ca2+ wave properties and sensitivity to modulation by calreticulin. We demonstrate by glucosidase inhibition and site-directed mutagenesis that a putative glycosylated residue (N1036) in SERCA2b is critical in determining both the selective targeting of calreticulin to SERCA2b and isoform functional differences. Calreticulin belongs to a novel class of lectin ER chaperones that modulate immature protein folding. In addition to this role, we suggest that these chaperones dynamically modulate the conformation of mature glycoproteins, thereby affecting their function.
Insights
Calreticulin modulates calcium (Ca2+) waves by interacting with specific sarcoendoplasmic reticulum Ca2+-ATPase (SERCA)2 isoforms. A key glycosylation site in SERCA2b influences calreticulin targeting and function.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Calreticulin (CRT) is an endoplasmic reticulum (ER) chaperone involved in protein folding and calcium (Ca2+) homeostasis.
- Inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ oscillations are crucial for cellular signaling and are regulated by Ca2+ uptake into the ER.
Purpose of the Study:
- To investigate the differential effects of calreticulin on alternatively spliced isoforms of sarcoendoplasmic reticulum Ca2+-ATPase (SERCA)2.
- To identify the molecular determinants of calreticulin's selective interaction with SERCA2 isoforms.
Main Methods:
- Overexpression of calreticulin in Xenopus laevis oocytes.
- Analysis of Ca2+ wave properties and oscillations.
- Site-directed mutagenesis and glucosidase inhibition to probe protein glycosylation and function.
Main Results:
- Calreticulin overexpression suppressed IP3-induced Ca2+ oscillations, suggesting inhibition of ER Ca2+ uptake.
- SERCA2a and SERCA2b isoforms exhibited distinct Ca2+ wave properties and sensitivities to calreticulin.
- A specific glycosylation site (N1036) in SERCA2b was critical for calreticulin's selective targeting and functional modulation.
Conclusions:
- Calreticulin differentially modulates SERCA2 isoforms, impacting Ca2+ signaling.
- Glycosylation plays a key role in the specific interaction between calreticulin and SERCA2b.
- Calreticulin's role extends beyond immature protein folding to dynamically modulating mature glycoprotein function.
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