Characteristics of irreversible ATP activation suggest that native skeletal ryanodine receptors can be phosphorylated via an endogenous CaMKII

Phosphorylation of skeletal muscle ryanodine receptor (RyR) calcium release channels by endogenous kinases incorporated into lipid bilayers with native sarcoplasmic reticulum vesicles was investigated during exposure to 2 mM cytoplasmic ATP. Activation of RyRs after 1-min exposure to ATP was reversible upon ATP washout. In contrast, activation after 5 to 8 min was largely irreversible: the small fall in activity with washout was significantly less than that after brief ATP exposure. The irreversible activation was reduced by acid phosphatase and was not seen after exposure to nonhydrolyzable ATP analogs. The data suggested that the channel complex was phosphorylated after addition of ATP and that phosphorylation reduced the RyR's sensitivity to ATP, adenosine, and Ca(2+). The endogenous kinase was likely to be a calcium calmodulin kinase II (CaMKII) because the CaMKII inhibitor KN-93 and an inhibitory peptide for CaMKII prevented the phosphorylation-induced irreversible activation. In contrast, phosphorylation effects remained unchanged with inhibitory peptides for protein kinase C and A. The presence of CaMKIIbeta in the SR vesicles was confirmed by immunoblotting. The results suggest that CaMKII is anchored to skeletal muscle RyRs and that phosphorylation by this kinase alters the enhancement of channel activity by ATP and Ca(2+).

Structural determinants for activation or inhibition of ryanodine receptors by basic residues in the dihydropyridine receptor II-III loop

The structures of peptide A, and six other 7-20 amino acid peptides corresponding to sequences in the A region (Thr671- Leu690) of the skeletal muscle dihydropyridine receptor II-III loop have been examined, and are correlated with the ability of the peptides to activate or inhibit skeletal ryanodine receptor calcium release channels. The peptides adopted either random coil or nascent helix-like structures, which depended upon the polarity of the terminal residues as well as the presence and ionisation state of two glutamate residues. Enhanced activation of Ca2+ release from sarcoplasmic reticulum, and activation of current flow through single ryanodine receptor channels (at -40 mV), was seen with peptides containing the basic residues 681Arg Lys Arg Arg Lys685, and was strongest when the residues were a part of an alpha-helix. Inhibition of channels (at +40 mV) was also seen with peptides containing the five positively charged residues, but was not enhanced in helical peptides. These results confirm the hypothesis that activation of ryanodine receptor channels by the II-III loop peptides requires both the basic residues and their participation in helical structure, and show for the first time that inhibition requires the basic residues, but is not structure-dependent. These findings imply that activation and inhibition result from peptide binding to separate sites on the ryanodine receptor.

A structural requirement for activation of skeletal ryanodine receptors by peptides of the dihydropyridine receptor II-III loop

The solution structures of three related peptides (A1, A2, and A9) corresponding to the Thr(671)-Leu(690) region of the skeletal muscle dihydropyridine receptor II-III loop have been investigated using nuclear magnetic resonance spectroscopy. Peptide A1, the native sequence, is less effective in activating ryanodine receptor calcium release channels than A2 (Ser(687) to Ala substitution). Peptide A9, Arg(681)-Ser(687), does not activate ryanodine receptors. A1 and A2 are helical from their N terminus to Lys(685) but are generally unstructured from Lys(685) to the C terminus. The basic residues Arg(681)-Lys(685), essential for A1 activation of ryanodine receptors, are located at the C-terminal end of the alpha-helix. Peptide A9 was found to be unstructured. Differences between A1 and A2 were observed in the C-terminal end of the helix (residues 681-685), which was less ordered in A1, and in the C-terminal region of the peptide, which exhibited greater flexibility in A1. Predicted low energy models suggest that an electrostatic interaction between the hydroxyl oxygen of Ser(687) and the guanidino moiety of Arg(683) is lost with the Ser(687)Ala substitution. The results show that the more structured peptides are more effective in activating ryanodine receptors.

A novel and de novo spontaneous point mutation (Glu271STOP) of the antithrombin gene results in a type I deficiency and thrombophilia

We describe a novel, de novo point mutation in one antithrombin (AT) allele resulting in type I AT deficiency and thrombophilia. Low plasma AT activity as well as low plasma AT antigen were documented in the propositus, but not in the parents, or in a male sibling. AT gene analysis by sequencing polymerase chain reaction-amplified genomic DNA from exon 5 of the propositus revealed a novel point mutation, GAG-->TAG at codon 271, resulting in a stop codon (Glu271STOP). This mutation was not demonstrable in the other members of his immediate family. DNA marker polymorphism analysis indicated the expected parentage. Based on allele frequency data for Caucasians in the United States the cumulative paternity index, or CPI, for the propositus and his father is 219,077. This corresponds to a probability of paternity of 99.9995% based on a prior probability of 50%. Included in this analysis is a linkage analysis of a trinucleotide repeat in intron 5 of the AT gene of the various family members, which also confirmed maternity and paternity. These studies provide documentation of the first spontaneous mutation of an AT gene in a thrombophilic individual, resulting in a type I AT deficiency.

Porin-type 1 proteins in sarcoplasmic reticulum and plasmalemma of striated muscle fibres

The location of porin-type 1 proteins in mammalian striated muscle has been assessed using immunogold electron microscopy with an anti-porin 31HL monoclonal antibody as the primary antibody. Gold particles were found on the mitochondrial outer membrane, the sarcoplasmic reticulum and plasmalemma in longitudinal sections of rat and rabbit skeletal muscle and rabbit and sheep cardiac muscle. The relative densities of gold particles in the mitochondrial outer membrane, sarcoplasmic reticulum and plasmalemma were 7:3:1 in white sternomastoid muscle, for example. Skeletal and cardiac sarcoplasmic reticulum vesicles, which had been fractionated by discontinuous sucrose density centrifugation, were subjected to SDS-polyacrylamide gel electrophoresis and Western blotting. The anti-porin 31HL monoclonal antibody detected a band of relative molecular mass (M(r)) 31,000 in all muscle sarcoplasmic reticulum vesicle fractions and also in liver mitochondria. The intensity of immunostaining of the sarcoplasmic reticulum fractions was 2.5-10% that of mitochondrial outer membranes per microgram of membrane protein blotted. Contamination of the sarcoplasmic reticulum fractions by mitochondrial outer membrane was < 0.75% as determined from the specific activity of monoamine oxidase. Thus, only a small part of the porin detected in sarcoplasmic reticulum vesicles can be attributed to mitochondrial contamination. These results show that porin-type1 immunoreactivity is not restricted to mitochondria but found in the sarcoplasmic reticulum and plasmalemma of both mammalian skeletal and cardiac muscle.