Regulation of the cardiac isoform of the ryanodine receptor by S-adenosyl-l-methionine
Gaboardi, Angela Kampfer
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Activity of the Ryanodine Receptor (RyR2) (aka cardiac Ca2+ release channel) plays a pivotal role in contraction of the heart. S-adenosyl-l-methionine (SAM) is a biological methyl group donor that has close structural similarity to ATP, an important physiological regulator of RyR2. This work provides evidence that SAM can act as a RyR2 regulatory ligand in a manner independent from its recognized role as a biological methyl group donor. RyR2 activation appears to arise from the direct interaction of SAM, via its adenosyl moiety, with the RyR2 adenine nucleotide binding sites. Because uncertainty remains regarding the structural motifs involved in RyR2 modulation by ATP and its metabolites, this finding has important implications for clarifying the structural basis of ATP regulation of RyR2. During the course of this project, direct measurements of single RyR2 activity revealed that SAM has distinct effects on RyR2 conductance. From the cytosolic side of the channel, SAM produced a single clearly resolved subconductance state. The effects of SAM on channel conductance were dependent on SAM concentration and membrane holding potential. A second goal of this work was to distinguish between the two possible mechanisms by which SAM could reduce RyR2 conductance: i) SAM interfering directly with ion permeation via binding within the conduction pathway (pore block), or ii) SAM binding a regulatory (or allosteric) site thereby stabilizing or inducing a reduced conductance conformation of the channel. It was determined that SAM does not directly interact with the RyR2 conduction pathway. To account for these observations an allosteric model for the effect of SAM on RyR2 conductance is proposed. According to this model, SAM binding stabilizes an inherent RyR2 subconductance conformation. The voltage dependence of the SAM related subconductance state is accounted for by direct effects of voltage on channel conformation which indirectly alter the affinity of RyR2 for SAM. Patterns in the transitions between RyR2 conductance states in the presence of SAM may provide insight into the structure-activity relationship of RyR2 which can aid in the development of therapeutic strategies targeting this channel.