Structural determinants of the IRF4/DNA homodimeric complex.

Interferon regulatory factor 4 (IRF4) is a key transcription factor (TF) in the regulation of immune cells, including B and T cells. It acts by binding DNA as both a homodimer and, in conjunction with other TFs, as a heterodimer. The choice of homo and heterodimeric/ DNA interactions is a critical aspect in the control of the transcriptional program and cell fate outcome. To characterize the nature of this interaction in the homodimeric complex, we have determined the crystal structure of the IRF4/ISRE homodimeric complex. We show that the complex formation is aided by a substantial DNA deformation with co-operative binding achieved exclusively through protein-DNA contact. This markedly contrasts with the heterodimeric form where DNA bound IRF4 is shown to physically interact with PU.1 TF to engage EICE1. We also show that the hotspot residues (Arg98, Cys99 and Asn102) contact both consensus and non-consensus sequences with the L1 loop exhibiting marked flexibility. Additionally, we identified that IRF4L116R, a mutant associated with chronic lymphocytic leukemia, binds more robustly to DNA thereby providing a rationale for the observed gain of function. Together, we demonstrate key structural differences between IRF4 homo and heterodimeric complexes, thereby providing molecular insights into IRF4-mediated transcriptional regulation.

Nuclear Transport of Respiratory Syncytial Virus Matrix Protein Is Regulated by Dual Phosphorylation Sites.

Respiratory syncytial virus (RSV) is a major cause of respiratory infections in infants and the elderly. Although the RSV matrix (M) protein has key roles in the nucleus early in infection, and in the cytoplasm later, the molecular basis of switching between the nuclear and cytoplasmic compartments is not known. Here, we show that protein kinase CK2 can regulate M nucleocytoplasmic distribution, whereby inhibition of CK2 using the specific inhibitor 4,5,6,7-tetrabromobenzo-triazole (TBB) increases M nuclear accumulation in infected cells as well as when ectopically expressed in transfected cells. We use truncation/mutagenic analysis for the first time to show that serine (S) 95 and threonine (T) 205 are key CK2 sites that regulate M nuclear localization. Dual alanine (A)-substitution to prevent phosphorylation abolished TBB- enhancement of nuclear accumulation, while aspartic acid (D) substitution to mimic phosphorylation at S95 increased nuclear accumulation. D95 also induced cytoplasmic aggregate formation, implying that a negative charge at S95 may modulate M oligomerization. A95/205 substitution in recombinant RSV resulted in reduced virus production compared with wild type, with D95/205 substitution resulting in an even greater level of attenuation. Our data support a model where unphosphorylated M is imported into the nucleus, followed by phosphorylation of T205 and S95 later in infection to facilitate nuclear export and cytoplasmic retention of M, respectively, as well as oligomerization/virus budding. In the absence of widely available, efficacious treatments to protect against RSV, the results raise the possibility of antiviral strategies targeted at CK2.

Glutathione transferase Omega 1 confers protection against azoxymethane-induced colorectal tumour formation.

Inflammatory bowel disease (IBD) is characterized by multiple alterations in cytokine expression and is a risk factor for colon cancer. The Omega class glutathione transferase GSTO1-1 regulates the release of the pro-inflammatory cytokines interleukin 1ß (IL-1ß) and interleukin 18 (IL-18) by deglutathionylating NEK7 in the NLRP3 inflammasome. When treated with azoxymethane and dextran sodium sulphate (AOM/DSS) as a model of IBD, Gsto1-/- mice were highly sensitive to colitis and showed a significant increase in the size and number of colon tumours compared with wild-type (WT) mice. Gsto1-/- mice treated with AOM/DSS had significantly lower serum IL-1ß and IL-18 levels as well as significantly decreased interferon (IFN)-γ, decreased pSTAT1 and increased pSTAT3 levels in the distal colon compared with similarly treated WT mice. Histologically, AOM/DSS treated Gsto1-/- mice showed increased active chronic inflammation with macrophage infiltration, epithelial dysplasia and invasive adenocarcinoma compared with AOM/DSS treated WT mice. Thus, this study shows that GSTO1-1 regulates IL-1ß and IL-18 activation and protects against colorectal cancer formation in the AOM/DSS model of IBD. The data suggest that while GSTO1-1 is a new target for the regulation of the NLRP3 inflammasome-associated cytokines IL-1ß and IL-18 by small molecule inhibitors, there is a possibility that anti-inflammatory drugs targeting these cytokines may potentiate colon cancer in some situations.

Association of FK506 binding proteins with RyR channels - effect of CLIC2 binding on sub-conductance opening and FKBP binding.

Ryanodine receptor (RyR) Ca2+ channels are central to striated muscle function and influence signalling in neurons and other cell types. Beneficially low RyR activity and maximum conductance opening may be stabilised when RyRs bind to FK506 binding proteins (FKBPs) and destabilised by FKBP dissociation, with submaximal opening during RyR hyperactivity associated with myopathies and neurological disorders. However, the correlation with submaximal opening is debated and quantitative evidence is lacking. Here, we have measured altered FKBP binding to RyRs and submaximal activity with addition of wild-type (WT) CLIC2, an inhibitory RyR ligand, or its H101Q mutant that hyperactivates RyRs, which probably causes cardiac and intellectual abnormalities. The proportion of sub-conductance opening increases with WT and H101Q CLIC2 and is correlated with reduced FKBP-RyR association. The sub-conductance opening reduces RyR currents in the presence of WT CLIC2. In contrast, sub-conductance openings contribute to excess RyR 'leak' with H101Q CLIC2. There are significant FKBP and RyR isoform-specific actions of CLIC2, rapamycin and FK506 on FKBP-RyR association. The results show that FKBPs do influence RyR gating and would contribute to excess Ca2+ release in this CLIC2 RyR channelopathy.

Functional and structural characterization of a novel malignant hyperthermia-susceptible variant of DHPR-ß1a subunit (CACNB1).

Malignant hyperthermia (MH) susceptibility has been recently linked to a novel variant of ß1a subunit of the dihydropyridine receptor (DHPR), a channel essential for Ca2+ regulation in skeletal muscle. Here we evaluate the effect of the mutant variant V156A on the structure/function of DHPR ß1a subunit and assess its role on Ca2+ metabolism of cultured myotubes. Using differential scanning fluorimetry, we show that mutation V156A causes a significant reduction in thermal stability of the Src homology 3/guanylate kinase core domain of ß1a subunit. Expression of the variant subunit in ß1-null mouse myotubes resulted in increased sensitivity to caffeine stimulation. Whole cell patch-clamp analysis of ß1a-V156A-expressing myotubes revealed a -2 mV shift in voltage dependence of channel activation, but no changes in Ca2+ conductance, current kinetics, or sarcoplasmic reticulum Ca2+ load were observed. Measurement of resting free Ca2+ and Na+ concentrations shows that both cations were significantly elevated in ß1a-V156A-expressing myotubes and that these changes were linked to increased rates of plasmalemmal Ca2+ entry through Na+/Ca2+ exchanger and/or transient receptor potential canonical channels. Overall, our data show that mutant variant V156A results in instability of protein subdomains of ß1a subunit leading to a phenotype of Ca2+ dysregulation that partly resembles that of other MH-linked mutations of DHPR α1S subunit. These data prove that homozygous expression of variant ß1a-V156A has the potential to be a pathological variant, although it may require other gene defects to cause a full MH phenotype.

Structural and biophysical analyses of the skeletal dihydropyridine receptor ß subunit ß1a reveal critical roles of domain interactions for stability.

Excitation-contraction (EC) coupling in skeletal muscle requires a physical interaction between the voltage-gated calcium channel dihydropyridine receptor (DHPR) and the ryanodine receptor Ca2+ release channel. Although the exact molecular mechanism that initiates skeletal EC coupling is unresolved, it is clear that both the α1 and ß subunits of DHPR are essential for this process. Here, we employed a series of techniques, including size-exclusion chromatography-multi-angle light scattering, differential scanning fluorimetry, and isothermal calorimetry, to characterize various biophysical properties of the skeletal DHPR ß subunit ß1a Removal of the intrinsically disordered N and C termini and the hook region of ß1a prevented oligomerization, allowing for its structural determination by X-ray crystallography. The structure had a topology similar to that of previously determined ß isoforms, which consist of SH3 and guanylate kinase domains. However, transition melting temperatures derived from the differential scanning fluorimetry experiments indicated a significant difference in stability of ∼2-3 °C between the ß1a and ß2a constructs, and the addition of the DHPR α1s I-II loop (α-interaction domain) peptide stabilized both ß isoforms by ∼6-8 °C. Similar to other ß isoforms, ß1a bound with nanomolar affinity to the α-interaction domain, but binding affinities were influenced by amino acid substitutions in the adjacent SH3 domain. These results suggest that intramolecular interactions between the SH3 and guanylate kinase domains play a role in the stability of ß1a while also providing a conduit for allosteric signaling events.

Nuclear PKC-θ facilitates rapid transcriptional responses in human memory CD4+ T cells through p65 and H2B phosphorylation.

Memory T cells are characterized by their rapid transcriptional programs upon re-stimulation. This transcriptional memory response is facilitated by permissive chromatin, but exactly how the permissive epigenetic landscape in memory T cells integrates incoming stimulatory signals remains poorly understood. By genome-wide ChIP-sequencing ex vivo human CD4(+) T cells, here, we show that the signaling enzyme, protein kinase C theta (PKC-θ) directly relays stimulatory signals to chromatin by binding to transcriptional-memory-responsive genes to induce transcriptional activation. Flanked by permissive histone modifications, these PKC-enriched regions are significantly enriched with NF-κB motifs in ex vivo bulk and vaccinia-responsive human memory CD4(+) T cells. Within the nucleus, PKC-θ catalytic activity maintains the Ser536 phosphorylation on the p65 subunit of NF-κB (also known as RelA) and can directly influence chromatin accessibility at transcriptional memory genes by regulating H2B deposition through Ser32 phosphorylation. Furthermore, using a cytoplasm-restricted PKC-θ mutant, we highlight that chromatin-anchored PKC-θ integrates activating signals at the chromatin template to elicit transcriptional memory responses in human memory T cells.

Probing interactions of Vpu from HIV-1 with amiloride-based compounds.

Viral ion channels or viroporins are short membrane proteins that participate in wide-ranging functions including virus replication and entry, assembly, and virus release. One such viroporin is the 81 amino acid residue Vpu protein derived from HIV-1. This protein consists of one transmembrane (TM) and two cytoplasmic helical domains, the former of which oligomerises to form cation-selective ion channels. In this study, we investigate the binding properties of amiloride compounds to Vpu embedded into liposomes using surface plasmon resonance (SPR). We explore the Vpu ion channel inhibitor, hexamethylene amiloride (HMA), as a molecular tool to examine the potential interactive role of key TM residues, Trp23, Ser24, and Glu29, in terms of positioning of these residues on the channel pore and the orientation of its constituent helices. The study provides experimental support that a direct interaction between Ser24 and HMA occurs and that this residue is most likely located in the channel pore. Mutation of Trp23 does not impact HMA affinity suggesting no direct involvement in binding and that this residue is lipid facing. These findings indicate that small molecules such as amilorides are capable of specifically interacting with Vpu ion channels. Although a correlation between ion channel and functional activity cannot be dismissed, alternative mechanisms involving protein-protein interactions may play an important role in the efficacy of these compounds.

Core skeletal muscle ryanodine receptor calcium release complex.

The core skeletal muscle ryanodine receptor (RyR1) calcium release complex extends through three compartments of the muscle fibre, linking the extracellular environment through the cytoplasmic junctional gap to the lumen of the internal sarcoplasmic reticulum (SR) calcium store. The protein complex is essential for skeletal excitation-contraction (EC)-coupling and skeletal muscle function. Its importance is highlighted by perinatal death if any one of the EC-coupling components are missing and by myopathies associated with mutation of any of the proteins. The proteins essential for EC-coupling include the DHPR α1S subunit in the transverse tubule membrane, the DHPR ß1a subunit in the cytosol and the RyR1 ion channel in the SR membrane. The other core proteins are triadin and junctin and calsequestrin, associated mainly with SR. These SR proteins are not essential for survival but exert structural and functional influences that modify the gain of EC-coupling and maintain normal muscle function. This review summarises our current knowledge of the individual protein/protein interactions within the core complex and their overall contribution to EC-coupling. We highlight significant areas that provide a continuing challenge for the field. Additional important components of the Ca2+ release complex, such as FKBP12, calmodulin, S100A1 and Stac3 are identified and reviewed elsewhere.

An α-helical C-terminal tail segment of the skeletal L-type Ca2+ channel ß1a subunit activates ryanodine receptor type 1 via a hydrophobic surface.

Excitation-contraction (EC) coupling in skeletal muscle depends on protein interactions between the transverse tubule dihydropyridine receptor (DHPR) voltage sensor and intracellular ryanodine receptor (RyR1) calcium release channel. We present novel data showing that the C-terminal 35 residues of the ß(1a) subunit adopt a nascent α-helix in which 3 hydrophobic residues align to form a hydrophobic surface that binds to RyR1 isolated from rabbit skeletal muscle. Mutation of the hydrophobic residues (L496, L500, W503) in peptide ß(1a)V490-M524, corresponding to the C-terminal 35 residues of ß(1a), reduced peptide binding to RyR1 to 15.2 ± 7.1% and prevented the 2.9 ± 0.2-fold activation of RyR1 by 10 nM wild-type peptide. An upstream hydrophobic heptad repeat implicated in ß(1a) binding to RyR1 does not contribute to RyR1 activation. Wild-type ß(1a)A474-A508 peptide (10 nM), containing heptad repeat and hydrophobic surface residues, increased RyR1 activity by 2.3 ± 0.2- and 2.2 ± 0.3-fold after mutation of the heptad repeat residues. We conclude that specific hydrophobic surface residues in the 35 residue ß(1a) C-terminus bind to RyR1 and increase channel activity in lipid bilayers and thus may support skeletal EC coupling.

Coexistence of two adamantane binding sites in the influenza A M2 ion channel.

The influenza A virus contains a proton-selective ion channel (M2) that is the target of the adamantane family of drug inhibitors. Two recently published studies relating to adamantane binding of the M2 ion channel using X-ray crystallography and solution NMR have reignited interest in the potential use of adamantanes in combating the spread of influenza A. However, these two studies propose different binding sites for the adamantane drugs with the X-ray M2/amantadine structure favoring an ion channel pore-binding model and the solution NMR M2/rimantadine structure suggesting the existence of a lipid-facing binding pocket. We conducted a series of surface plasmon resonance (SPR) experiments designed to accurately measure the affinity of amantadine and rimantadine to M2 ion channels embedded in 1,2-dimyristoyl-sn-glycero-phosphocholine (DMPC) liposomes. We find that this class of drug is capable of binding M2 with two different affinities in the order of 10(-4) and 10(-7) M, suggesting that both proposed binding sites are feasible. Furthermore, by examining drug binding to M2 mutant constructs (V27A, S31N, and D44A), it was possible to probe the location of the two binding sites. We show that a high-affinity binding site corresponds to the M2 ion channel pore whereas the secondary, low-affinity binding site can be attributed to the lipid face of the pore. These SPR results are in excellent agreement with the most recent solid-state NMR study of amantadine-bound M2 in lipid bilayers and provide independent support that the ion channel pore-binding site is responsible for the pharmacological activity elicited by the adamantane drugs.

A maladaptive feedback mechanism between the extracellular matrix and cytoskeleton contributes to hypertrophic cardiomyopathy pathophysiology.

Hypertrophic cardiomyopathy is an inherited disorder due to mutations in contractile proteins that results in a stiff, hypercontractile myocardium. To understand the role of cardiac stiffness in disease progression, here we create an in vitro model of hypertrophic cardiomyopathy utilizing hydrogel technology. Culturing wild-type cardiac myocytes on hydrogels with a Young's Moduli (stiffness) mimicking hypertrophic cardiomyopathy myocardium is sufficient to induce a hypermetabolic mitochondrial state versus myocytes plated on hydrogels simulating healthy myocardium. Significantly, these data mirror that of myocytes isolated from a murine model of human hypertrophic cardiomyopathy (cTnI-G203S). Conversely, cTnI-G203S myocyte mitochondrial function is completely restored when plated on hydrogels mimicking healthy myocardium. We identify a mechanosensing feedback mechanism between the extracellular matrix and cytoskeletal network that regulates mitochondrial function under healthy conditions, but participates in the progression of hypertrophic cardiomyopathy pathophysiology resulting from sarcomeric gene mutations. Importantly, we pinpoint key 'linker' sites in this schema that may represent potential therapeutic targets.

Transcriptional reprogramming by mutated IRF4 in lymphoma.

Disease-causing mutations in genes encoding transcription factors (TFs) can affect TF interactions with their cognate DNA-binding motifs. Whether and how TF mutations impact upon the binding to TF composite elements (CE) and the interaction with other TFs is unclear. Here, we report a distinct mechanism of TF alteration in human lymphomas with perturbed B cell identity, in particular classic Hodgkin lymphoma. It is caused by a recurrent somatic missense mutation c.295 T > C (p.Cys99Arg; p.C99R) targeting the center of the DNA-binding domain of Interferon Regulatory Factor 4 (IRF4), a key TF in immune cells. IRF4-C99R fundamentally alters IRF4 DNA-binding, with loss-of-binding to canonical IRF motifs and neomorphic gain-of-binding to canonical and non-canonical IRF CEs. IRF4-C99R thoroughly modifies IRF4 function by blocking IRF4-dependent plasma cell induction, and up-regulates disease-specific genes in a non-canonical Activator Protein-1 (AP-1)-IRF-CE (AICE)-dependent manner. Our data explain how a single mutation causes a complex switch of TF specificity and gene regulation and open the perspective to specifically block the neomorphic DNA-binding activities of a mutant TF.

A multimorphic mutation in IRF4 causes human autosomal dominant combined immunodeficiency.

Interferon regulatory factor 4 (IRF4) is a transcription factor (TF) and key regulator of immune cell development and function. We report a recurrent heterozygous mutation in IRF4, p.T95R, causing an autosomal dominant combined immunodeficiency (CID) in seven patients from six unrelated families. The patients exhibited profound susceptibility to opportunistic infections, notably Pneumocystis jirovecii, and presented with agammaglobulinemia. Patients' B cells showed impaired maturation, decreased immunoglobulin isotype switching, and defective plasma cell differentiation, whereas their T cells contained reduced TH17 and TFH populations and exhibited decreased cytokine production. A knock-in mouse model of heterozygous T95R showed a severe defect in antibody production both at the steady state and after immunization with different types of antigens, consistent with the CID observed in these patients. The IRF4T95R variant maps to the TF's DNA binding domain, alters its canonical DNA binding specificities, and results in a simultaneous multimorphic combination of loss, gain, and new functions for IRF4. IRF4T95R behaved as a gain-of-function hypermorph by binding to DNA with higher affinity than IRF4WT. Despite this increased affinity for DNA, the transcriptional activity on IRF4 canonical genes was reduced, showcasing a hypomorphic activity of IRF4T95R. Simultaneously, IRF4T95R functions as a neomorph by binding to noncanonical DNA sites to alter the gene expression profile, including the transcription of genes exclusively induced by IRF4T95R but not by IRF4WT. This previously undescribed multimorphic IRF4 pathophysiology disrupts normal lymphocyte biology, causing human disease.

Novel folding and stability defects cause a deficiency of human glutathione transferase omega 1.

The polymorphic deletion of Glu-155 from human glutathione transferase omega1 (GSTO1-1) occurs in most populations. Although the recombinant ΔGlu-155 enzyme expressed in Escherichia coli is active, the deletion causes a deficiency of the active enzyme in vivo. The crystal structure and the folding/unfolding kinetics of the ΔGlu-155 variant were determined in order to investigate the cause of the rapid loss of the enzyme in human cells. The crystal structure revealed altered packing around the Glu-155 deletion, an increase in the predicted solvent-accessible area and a corresponding reduction in the buried surface area. This increase in solvent accessibility was consistent with an elevated Stern-Volmer constant. The unfolding of both the wild type and ΔGlu-155 enzyme in urea is best described by a three-state model, and there is evidence for the more pronounced population of an intermediate state by the ΔGlu-155 enzymes. Studies using intrinsic fluorescence revealed a free energy change around 14.4 kcal/mol for the wild type compared with around 8.6 kcal/mol for the ΔGlu-155 variant, which indicates a decrease in stability associated with the Glu-155 deletion. Urea induced unfolding of the wild type GSTO1-1 was reversible through an initial fast phase followed by a second slow phase. In contrast, the ΔGlu-155 variant lacks the slow phase, indicating a refolding defect. It is possible that in some conditions in vivo, the increased solvent-accessible area and the low stability of the ΔGlu-155 variant may promote its unfolding, whereas the refolding defect limits its refolding, resulting in GSTO1-1 deficiency.

Cyclization of the intrinsically disordered α1S dihydropyridine receptor II-III loop enhances secondary structure and in vitro function.

A key component of excitation contraction (EC) coupling in skeletal muscle is the cytoplasmic linker (II-III loop) between the second and third transmembrane repeats of the α(1S) subunit of the dihydropyridine receptor (DHPR). The II-III loop has been previously examined in vitro using a linear II-III loop with unrestrained N- and C-terminal ends. To better reproduce the loop structure in its native environment (tethered to the DHPR transmembrane domains), we have joined the N and C termini using intein-mediated technology. Circular dichroism and NMR spectroscopy revealed a structural shift in the cyclized loop toward a protein with increased α-helical and ß-strand structure in a region of the loop implicated in its in vitro function and also in a critical region for EC coupling. The affinity of binding of the II-III loop binding to the SPRY2 domain of the skeletal ryanodine receptor (RyR1) increased 4-fold, and its ability to activate RyR1 channels in lipid bilayers was enhanced 3-fold by cyclization. These functional changes were predicted consequences of the structural enhancement. We suggest that tethering the N and C termini stabilized secondary structural elements in the DHPR II-III loop and may reflect structural and dynamic characteristics of the loop that are inherent in EC coupling.

The molecular basis for the development of adult T-cell leukemia/lymphoma in patients with an IRF4K59R mutation.

Interferon regulatory factor 4 (IRF4) is an essential regulator in the development of many immune cells, including B- and T-cells and has been implicated directly in numerous hematological malignancies, including adult T-cell leukemia/lymphoma (ATLL). Recently, an activating mutation in the DNA-binding domain of IRF4 (IRF4K59R ) was found as a recurrent somatic mutation in ATLL patients. However, it remains unknown how this mutation gives rise to the observed oncogenic effect. To understand the mode of IRF4K59R -mediated gain of function in ATLL pathogenesis, we have determined the structural and affinity basis of IRF4K59R /DNA homodimer complex using X-ray crystallography and surface plasmon resonance. Our study shows that arginine substitution (R59) results in the reorientation of the side chain, enabling the guanidium group to interact with the phosphate backbone of the DNA helix. This markedly contrasts with the IRF4WT wherein the K59 interacts exclusively with DNA bases. Further, the arginine mutation causes enhanced DNA bending, enabling the IRF4K59R to interact more robustly with known DNA targets, as evidenced by increased binding affinity of the protein-DNA complex. Together, we demonstrate how key structural features underpin the basis for this activating mutation, thereby providing a molecular rationale for IRF4K59R -mediated ATLL development.

Molecular interactions of STAC proteins with skeletal muscle dihydropyridine receptor and excitation-contraction coupling.

Excitation-contraction coupling (ECC) is the physiological process in which an electrical signal originating from the central nervous system is converted into muscle contraction. In skeletal muscle tissue, the key step in the molecular mechanism of ECC initiated by the muscle action potential is the cooperation between two Ca2+ channels, dihydropyridine receptor (DHPR; voltage-dependent L-type calcium channel) and ryanodine receptor 1 (RyR1). These two channels were originally postulated to communicate with each other via direct mechanical interactions; however, the molecular details of this cooperation have remained ambiguous. Recently, it has been proposed that one or more supporting proteins are in fact required for communication of DHPR with RyR1 during the ECC process. One such protein that is increasingly believed to play a role in this interaction is the SH3 and cysteine-rich domain-containing protein 3 (STAC3), which has been proposed to bind a cytosolic portion of the DHPR α1S subunit known as the II-III loop. In this work, we present direct evidence for an interaction between a small peptide sequence of the II-III loop and several residues within the SH3 domains of STAC3 as well as the neuronal isoform STAC2. Differences in this interaction between STAC3 and STAC2 suggest that STAC3 possesses distinct biophysical features that are potentially important for its physiological interactions with the II-III loop. Therefore, this work demonstrates an isoform-specific interaction between STAC3 and the II-III loop of DHPR and provides novel insights into a putative molecular mechanism behind this association in the skeletal muscle ECC process.

The ß(1a) subunit of the skeletal DHPR binds to skeletal RyR1 and activates the channel via its 35-residue C-terminal tail.

Although it has been suggested that the C-terminal tail of the ß(1a) subunit of the skeletal dihyropyridine receptor (DHPR) may contribute to voltage-activated Ca(2+) release in skeletal muscle by interacting with the skeletal ryanodine receptor (RyR1), a direct functional interaction between the two proteins has not been demonstrated previously. Such an interaction is reported here. A peptide with the sequence of the C-terminal 35 residues of ß(1a) bound to RyR1 in affinity chromatography. The full-length ß(1a) subunit and the C-terminal peptide increased [(3)H]ryanodine binding and RyR1 channel activity with an AC(50) of 450-600 pM under optimal conditions. The effect of the peptide was dependent on cytoplasmic Ca(2+), ATP, and Mg(2+) concentrations. There was no effect of the peptide when channel activity was very low as a result of Mg(2+) inhibition or addition of 100 nM Ca(2+) (without ATP). Maximum increases were seen with 1-10 µM Ca(2+), in the absence of Mg(2+) inhibition. A control peptide with the C-terminal 35 residues in a scrambled sequence did not bind to RyR1 or alter [(3)H]ryanodine binding or channel activity. This high-affinity in vitro functional interaction between the C-terminal 35 residues of the DHPR ß(1a) subunit and RyR1 may support an in vivo function of ß(1a) during voltage-activated Ca(2+) release.