Pharmacological clues to calmodulin-mediated activation of skeletal ryanodine receptor using [3H]-ryanodine binding

Two potential calmodulin-binding sequences in the ryanodine receptor contribute to a mobile, intra-subunit calmodulin-binding domain

Calmodulin (CaM), a 16 kDa ubiquitous calcium-sensing protein, is known to bind tightly to the calcium release channel/ryanodine receptor (RyR), and modulate RyR function. CaM binding studies using RyR fragments or synthetic peptides have revealed the presence of multiple, potential CaM-binding regions in the primary sequence of RyR. In the present study, we inserted GFP into two of these proposed CaM-binding sequences and mapped them onto the three-dimensional structure of intact cardiac RyR2 by cryo-electron microscopy. Interestingly, we found that the two potential CaM-binding regions encompassing, Arg3595 and Lys4269, respectively, are in close proximity and are adjacent to the previously mapped CaM-binding sites. To monitor the conformational dynamics of these CaM-binding regions, we generated a fluorescence resonance energy transfer (FRET) pair, a dual CFP- and YFP-labeled RyR2 (RyR2R3595-CFP/K4269-YFP) with CFP inserted after Arg3595 and YFP inserted after Lys4269. We transfected HEK293 cells with the RyR2R3595-CFP/K4269-YFP cDNA, and examined their FRET signal in live cells. We detected significant FRET signals in transfected cells that are sensitive to the channel activator caffeine, suggesting that caffeine is able to induce conformational changes in these CaM-binding regions. Importantly, no significant FRET signals were detected in cells co-transfected with cDNAs encoding the single CFP (RyR2R3595-CFP) and single YFP (RyR2K4269-YFP) insertions, indicating that the FRET signal stemmed from the interaction between R3595-CFP and K4269-YFP that are in the same RyR subunit. These observations suggest that multiple regions in the RyR2 sequence may contribute to an intra-subunit CaM-binding pocket that undergoes conformational changes during channel gating.

Ryanodine receptor calcium release channels: lessons from structure-function studies

Ryanodine receptors (RyRs) are the largest known ion channels. They are Ca(2+) release channels found primarily on the sarcoplasmic reticulum of myocytes. Several hundred mutations in RyRs are associated with skeletal or cardiomyocyte disease in humans. Many of these mutations can now be mapped onto the high resolution structures of individual RyR domains and on full-length tetrameric cryo-electron microscopy structures. A closely related Ca(2+) release channel, the inositol 1,4,5-trisphospate receptor (IP3 R), shows a conserved structural architecture at the N-terminus, suggesting that both channels evolved from an ancestral unicellular RyR/IP3 R. The functional insights provided by recent structural studies for both channels will aid in the development of rationale treatments for a myriad of Ca(2+)-signaled malignancies.

Calmodulin-binding locations on the skeletal and cardiac ryanodine receptors

Ryanodine receptor types 1 (RyR1) and 2 (RyR2) are calcium release channels that are highly enriched in skeletal and cardiac muscle, respectively, where they play an essential role in excitation-contraction coupling. Apocalmodulin (apo-CaM) weakly activates RyR1 but inhibits RyR2, whereas Ca(2+)-calmodulin inhibits both isoforms. Previous cryo-EM studies showed distinctly different binding locations on RyR1 for the two states of CaM. However, recent studies employing FRET appear to challenge these findings. Here, using cryo-EM, we have determined that a CaM mutant that is incapable of binding calcium binds to RyR1 at the apo site, regardless of the calcium concentration. We have also re-determined the location of RyR1-bound Ca(2+)-CaM using uniform experimental conditions. Our results show the existence of the two overlapping but distinct binding sites for CaM in RyR1 and imply that the binding location switch is due to Ca(2+) binding to CaM, as opposed to direct effects of Ca(2+) on RyR1. We also discuss explanations that could resolve the apparent conflict between the cryo-EM and FRET results. Interestingly, apo-CaM binds to RyR2 at a similar binding location to that of Ca(2+)-CaM on RyR1, in seeming agreement with the inhibitory effects of these two forms of CaM on their respective receptors.

Post-natal developmental expression of alphaKAP splice variants in rabbit fast-twitch and slow-twitch skeletal muscle

AlphaKAP is a non-kinase product of a gene within alpha CaMKII gene, that anchors catalytic subunits to skeletal muscle sarcoplasmic reticulum. alphaKAP gene expresses two distinct mRNA transcripts by alternative splicing in rabbit fast muscle, rather than one, as an apparently fiber-type specific and species-specific property. We report on the expression profile of alphaKAP gene in fast and slow muscles of adult rats and in rabbit post-natal developing muscles, by RT-PCR analysis of muscle RNA, together with Western blot analysis of expressed alphaKAP protein isoforms. We show that alphaKAP isoform predominantly expressed in muscle of newborn rabbits, corresponds to the fast-adult isoform. In fast muscle, the transition from neonatal isoform to the common adult isoform takes place early after birth, and is never complete, unlike in slow muscle. Relevant to these findings, is the observation that neonatal/embryonic alphaKAP mRNA is highly expressed in rat myotubes grown in vitro. Together, our findings suggest that alphaKAP splicing pathways are primarily modulated according to muscle developmental stage.

Two splice variants of CaMKII-anchoring protein are present in the sarcoplasmic reticulum of rabbit fast-twitch muscle

Anchoring protein alphaKAP targets calmodulin kinase II (CaMKII) to the sarcoplasmic reticulum (SR), and in the rabbit is a substrate of CaMKII itself in fast-twitch, but not in slow-twitch muscle. This work was aimed at elucidating the molecular basis for differential phosphorylation of alphaKAP. Here we show that two, immunologically related, size forms (23 and 21 kDa) of alphaKAP are present in fast-twitch muscle SR in a 3:1 stoichiometry. Phosphorylation experiments identified the shorter form as the CaMKII specific substrate. Both forms are shown to be stably integrated into the holoenzyme. Two splice variants of alphaKAP were found in rabbit fast-twitch muscle and only one in slow-twitch muscle, using RT-PCR. Mobilities on SDS-PAGE are those expected. The shorter splice variants lacks the 33-nucleotide sequence inserted by alternative splicing present in full-length alphaKAP, akin to differences between variants A and B of brain alphaCaMKII. The absence of the 11-amino acid sequence creates a novel CaMKII phosphorylation site. Taken together our results show that alternative splicing regulates alphaKAP phosphorylation in a fiber-type specific manner.

Phosphorylation of the triadin cytoplasmic domain by CaM protein kinase in rabbit fast-twitch muscle sarcoplasmic reticulum

Skeletal muscle triadin is a sarcoplasmic reticulum (SR) membrane protein that had been shown to interact structurally and functionally at the cytoplasmic domain (amino acid residues 1-47) with the ryanodine receptor (RyR1), and to undergo phosphorylation by endogenous calmodulin protein kinase (CaM K II) in isolated terminal cisternae from rabbit fast-twitch muscle. Here we show that triadin cytoplasmic domain expressed as glutathione-S-transferase fusion protein, is a substrate of the protein kinase. This finding is corroborated by identification of a specific consensus sequence in the deduced amino sequence between residue 34 and 37 of triadin. Confirming the regulatory features of CaM K II, we show the phosphorylation of triadin cytoplasmic segment by the kinase, when converted to the autonomous form. We propose that triadin modulates RyR1 in a phosphorylation-dependent manner.