Role of the Sequence Surrounding Predicted Transmembrane Helix M4 in Membrane Association and Function of the Ca2+ Release Channel of Skeletal Muscle Sarcoplasmic Reticulum (Ryanodine Receptor Isoform 1)

Abstract

The role of the sequence surrounding M4 in ryanodine receptors (RyR) in membrane association and function was investigated. This sequence contains a basic, 19-amino acid M3/M4 loop, a hydrophobic 44 -49 amino acid sequence designated M4 (or M4a/M4b), and a hydrophilic M4/M5 loop. Enhanced green fluorescent protein (EGFP) was inserted into RyR1 and truncated just after the basic sequence, just after M4, within the M4/M5 loop, just before M5 and just after M5. The A52 epitope was inserted into RyR2 and truncated just after M4a. Analysis of these constructs ruled out a M3/M4 transmembrane hairpin and narrowed the region of membrane association to M4a/M4b. EGFP inserted between M4a and M4b in full-length RyR2 was altered conformationally, losing fluorescence and gaining trypsin sensitivity. Although it was accessible to an antibody from the cytosolic side, tryptic fragments were membrane-bound. The expressed protein containing EGFP retained caffeine-induced Ca 2؉ release channel function. These results suggest that M4a/M4b either forms a transmembrane hairpin or associates in an unorthodox fashion with the cytosolic leaflet of the membrane, possibly involving the basic M3/M4 loop. The expression of a mutant RyR1, ⌬4274 -4535, deleted in the sequence surrounding both M3 and M4, restored robust, voltagegated L-type Ca 2؉ currents and Ca 2؉ transients in dyspedic myotubes, demonstrating that this sequence is not required for either orthograde (DHPR activation of sarcoplasmic reticulum Ca 2؉ release) or retrograde (RyR1 increase in DHPR Ca 2؉ channel activity) signals of excitation-contraction coupling. Maximal amplitudes of L-currents and Ca 2؉ transients with ⌬4274 -4535 were larger than with wild-type RyR1, and voltage-gated sarcoplasmic reticulum Ca 2؉ release was more sensitive to activation by sarcolemmal voltage sensors. Thus, this region may act as a negative regulatory module that increases the energy barrier for Ca 2؉ release channel opening.In our first analysis of the transmembrane (TM) 1 sequences in the ryanodine receptor isoform 1 (RyR1), we predicted that five TM hairpin loops were present in the C terminus of RyR1, among which M3 was formed by amino acids 4277-4300 and M4 was formed by amino acids 4342-4362 (1). Later sequencing of other RyR isoforms (2, 3) showed that the sequence surrounding M3 and M4 is among the most diverse, leading to designation of the sequence between amino acids 4254 and 4631 as divergent region 1 or D1 (4). M3 is not hydrophobic in RyR3, although it is in RyR1 and RyR2. However, the hydrophobicity of a 41-amino acid sequence (5, 6), which includes predicted M4, is conserved in all three isoforms. A hydrophobic sequence of this length is considered to be sufficient to form a TM hairpin loop by itself. Indeed, the TMHMM2.0 program (7) predicts that this sequence of up to 49 amino acids will form a TM hairpin loop in each of RyR1, RyR2, and RyR3 (Fig. 1). The sequence separating the C terminus of M3 and the beginning of the hydrophobic M4 sequence is hydrophilic and very basic (1...

Structural Determinants of Skeletal Muscle Ryanodine Receptor Gating*

“…1B) further defined the boundaries of the six TM segments of RyR1. The boundaries of the TM segments combined with the known luminal or cytosolic location of intervening loops in between the TM segments (1,48) enabled us to propose a TM topology for RyR1 (Fig. 1C), which underscores its similarity to the known structures of 6-TM cation channels (15, 23, 49).…”

“…1A) in the RyR1 C-terminal region (residues 4561-4948; UniProt accession number P11716). The presence of the six membrane-spanning segments is sufficient to support RyR1 ion channel activity as seen by single channel recordings of tryptic fragments (47) and deletion of the preceding putative TM segments (48). Overlaying the TM prediction on secondary structure prediction (Fig.…”

Targeting and Retention of Type 1 Ryanodine Receptors to the Endoplasmic Reticulum

“…1A. Substantial additional attempts to resolve the topology of this region (26) failed to demonstrate the presence of a pair of TMD before TMD1, suggesting that the region may be associated with the membrane without integrating into it. The latter is consistent also with three-dimensional reconstructions of cryoelectron microscopic images of RyR1.…”

The Gln4863Ala Mutation within a Putative, Pore-Lining Trans-Membrane Helix of the Cardiac Ryanodine Receptor Channel Alters Both the Kinetics of Ryanoid Interaction and the Subsequent Fractional Conductance

“…Channel conduction has been localized to the C-terminal fifth of the protein by truncation (Bhat et al, 1997). Bioinformatics (Shah and Sowdhamini, 2001;Williams et al, 2001) and extensive mu-tational studies have further identified putative channel elements of RyRs (Zhao et al, 1999;Gao et al, 2000;Du et al, 2001Du et al, , 2004Chen et al, 2002;Wang et al, 2003Wang et al, , 2004. Most recently, a 3D model of the RyR pore (Welch et al, 2004) has been proposed derived from structural analogies with the known X-ray structure of the prokaryotic K ϩ channel KcsA (Doyle et al, 1998).…”

“…Extensive alanine scanning of these putative pore-forming domains of RyRs has identified several residues that abolish [ 3 H]ryanodine binding (Zhao et al, 1999;Gao et al, 2000;Du et al, 2001Du et al, , 2004Chen et al, 2002;Wang et al, 2003Wang et al, , 2004. Those that retain caffeine sensitivity, implying a lack of gross distortion of structure, but abolish ryanodine-sensitivity indicate a clear effect on the ryanodine binding site (Wang et al, 2003).…”