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Selective binding of FKBP12.6 by the cardiac ryanodine receptor
A P Timerman1, H Onoue, H B Xin
1Department of Molecular Biology Vanderbilt University, Nashville, Tennessee 37235, USA.
Insights
FKBP12 and FKBP12.6 binding to calcium release channels (CRC) differs between skeletal and cardiac muscle. This selective interaction explains distinct CRC structures and may influence muscle excitation-contraction coupling.
Area of Science:
- Molecular biology
- Muscle physiology
- Biochemistry
Background:
- Calcium release channels (CRC)/ryanodine receptors (RyRs) in skeletal (Sk) and cardiac (C) muscle sarcoplasmic reticulum (SR) are complexes involving RyR protomers and FKBP proteins.
- FKBP12 and FKBP12.6 are isoforms of the 12-kDa FK506-binding protein, showing sequence similarities to RyR protomers.
Purpose of the Study:
- To investigate the interaction of FKBP12 and FKBP12.6 with skeletal and cardiac CRCs.
- To elucidate the role of FKBP isoforms in the structural differences and functional modulation of CRCs in different muscle types.
Main Methods:
- Utilized 35S-labeled FKBP12 and FKBP12.6 as probes to study CRC interactions.
- Performed experiments involving FK506/FK590 to assess FKBP dissociation.
- Investigated FKBP binding and exchange with FKBP-stripped and endogenously bound CRCs.
Main Results:
- FK506 dissociates FKBP12.6 from cardiac CRCs, similar to FKBP12 in skeletal CRCs.
- Both FKBP isoforms bind to and exchange with skeletal CRCs, but only FKBP12.6 binds/exchanges with cardiac CRCs.
- Cardiac CRCs are isolated with FKBP12.6, while skeletal CRCs are isolated with FKBP12, despite FKBP12 presence in both muscle types.
Conclusions:
- Selective binding of FKBP isoforms to CRCs explains their differential association with skeletal and cardiac muscle SR.
- Unlike skeletal muscle, removal of FKBP from cardiac CRCs does not activate channel activity, suggesting distinct regulatory mechanisms.
- Differential FKBP action may represent a fundamental difference in excitation-contraction coupling modulation between heart and skeletal muscle.
Abstract:
The calcium release channels (CRC)/ryanodine receptors of skeletal (Sk) and cardiac (C) muscle sarcoplasmic reticulum (SR) are hetero-oligomeric complexes with the structural formulas (ryanodine recepter (RyR)1 protomer)4(FKBP12)4 and (RyR2 protomer)4(FKBP12.6)4, respectively, where FKBP12 and FKBP12.6 are isoforms of the 12-kDa receptor for the immunosuppressant drug FK506. The sequence similarity between the RyR protomers and FKBP12 isoforms is 63 and 85%, respectively. Using 35S-labeled FKBP12 and 35S-labeled FKBP12.6 as probes to study the interaction with CRC, we find that: 1) analogous to its action in skeletal muscle sarcoplasmic reticulum (SkMSR), FK506 (or analog FK590) dissociates FKBP12.6 from CSR; 2) both FKBP isoforms bind to FKBP-stripped SkMSR and exchange with endogenously bound FKBP12 of SkMSR; and 3) by contrast, only FKBP12. 6 exchanges with endogenously bound FKBP12.6 or rebinds to FKBP-stripped CSR. This selective binding appears to explain why the cardiac CRC is isolated as a complex with FKBP12.6, whereas the skeletal muscle CRC is isolated as a complex with FKBP12, although only FKBP12 is detectable in the myoplasm of both muscle types. Also, in contrast to the activation of the channel by removal of FKBP from skeletal muscle, no activation is detected in CRC activity in FKBP-stripped CSR. This differential action of FKBP may reflect a fundamental difference in the modulation of excitation-contraction coupling in heart versus skeletal muscle.