Journal article
Calsequestrin depolymerizes when calcium is depleted in the sarcoplasmic reticulum of working muscle
Proceedings of the National Academy of Sciences - PNAS, Vol.114(4), pp.E638-E647
PNAS Plus
01/24/2017
Handle:
https://hdl.handle.net/2376/110312
PMCID: PMC5278470
PMID: 28069951
Abstract
We show that calsequestrin, the main Ca
2+
storing protein of muscle, is polymerized inside the sarcoplasmic reticulum (SR) and its mobility increases greatly upon SR depletion, indicating depolymerization. Deep depletion causes massive calsequestrin migration and radical SR remodeling, often accompanied by a surge in intra-SR free Ca
2+
. The changes in calsequestrin polymerization observed in aqueous solutions therefore also occur in vivo. These changes help explain some uniquely advantageous properties of the SR as a source of calcium for contractile activation. The results support untested hypotheses about additional calsequestrin roles in the control of channel gating and facilitation of calcium flux. They also provide insights on the consequences of calsequestrin mutations for functional competence and structural stability of skeletal and cardiac muscle.
Calsequestrin, the only known protein with cyclical storage and supply of calcium as main role, is proposed to have other functions, which remain unproven. Voluntary movement and the heart beat require this calcium flow to be massive and fast. How does calsequestrin do it? To bind large amounts of calcium in vitro, calsequestrin must polymerize and then depolymerize to release it. Does this rule apply inside the sarcoplasmic reticulum (SR) of a working cell? We answered using fluorescently tagged calsequestrin expressed in muscles of mice. By FRAP and imaging we monitored mobility of calsequestrin as [Ca
2+
] in the SR--measured with a calsequestrin-fused biosensor--was lowered. We found that calsequestrin is polymerized within the SR at rest and that it depolymerized as [Ca
2+
] went down: fully when calcium depletion was maximal (a condition achieved with an SR calcium channel opening drug) and partially when depletion was limited (a condition imposed by fatiguing stimulation, long-lasting depolarization, or low drug concentrations). With fluorescence and electron microscopic imaging we demonstrated massive movements of calsequestrin accompanied by drastic morphological SR changes in fully depleted cells. When cells were partially depleted no remodeling was found. The present results support the proposed role of calsequestrin in termination of calcium release by conformationally inducing closure of SR channels. A channel closing switch operated by calsequestrin depolymerization will limit depletion, thereby preventing full disassembly of the polymeric calsequestrin network and catastrophic structural changes in the SR.
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Details
- Title
- Calsequestrin depolymerizes when calcium is depleted in the sarcoplasmic reticulum of working muscle
- Creators
- Carlo Manno - Section of Cellular SignalingLourdes C Figueroa - Section of Cellular SignalingDirk Gillespie - Section of Cellular SignalingRobert Fitts - Department of BiologyChulHee Kang - Department of ChemistryClara Franzini-Armstrong - Department of Cell and Developmental BiologyEduardo Rios - Section of Cellular Signaling
- Publication Details
- Proceedings of the National Academy of Sciences - PNAS, Vol.114(4), pp.E638-E647
- Academic Unit
- Chemistry, Department of
- Series
- PNAS Plus
- Publisher
- National Academy of Sciences
- Grant note
- AR054098 / HHS | NIH | National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) GM111254 / HHS | NIH | National Institute of General Medical Sciences (NIGMS)
- Identifiers
- 99900547034401842
- Language
- English
- Resource Type
- Journal article