In Gram-negative bacteria and eukaryotic organelles, β-barrel proteins of the outer membrane protein 85–two-partner secretion B (Omp85-TpsB) superfamily are essential components of protein transport machineries. The TpsB transporter FhaC mediates the secretion of Bordetella pertussis filamentous hemagglutinin (FHA). We report the 3.15 Å crystal structure of FhaC. The transporter comprises a 16-stranded β barrel that is occluded by an N-terminal α helix and an extracellular loop and a periplasmic module composed of two aligned polypeptide-transport–associated (POTRA) domains. Functional data reveal that FHA binds to the POTRA 1 domain via its N-terminal domain and likely translocates the adhesin-repeated motifs in an extended hairpin conformation, with folding occurring at the cell surface. General features of the mechanism obtained here are likely to apply throughout the superfamily.

Increased sarcoplasmic reticulum (SR) Ca²⁺ leak via the cardiac ryanodine receptor/calcium release channel (RyR2) is thought to play a role in heart failure (HF) progression. Inhibition of this leak is an emerging therapeutic strategy. To explore the role of chronic PKA phosphorylation of RyR2 in HF pathogenesis and treatment, we generated a knockin mouse with aspartic acid replacing serine 2808 (mice are referred to herein as RyR2-S2808D^+/+ mice). This mutation mimics constitutive PKA hyperphosphorylation of RyR2, which causes depletion of the stabilizing subunit FKBP12.6 (also known as calstabin2), resulting in leaky RyR2. RyR2-S2808D^+/+ mice developed age-dependent cardiomyopathy, elevated RyR2 oxidation and nitrosylation, reduced SR Ca²⁺ store content, and increased diastolic SR Ca²⁺ leak. After myocardial infarction, RyR2-S2808D^+/+ mice exhibited increased mortality compared with WT littermates. Treatment with S107, a 1,4-benzothiazepine derivative that stabilizes RyR2-calstabin2 interactions, inhibited the RyR2-mediated diastolic SR Ca²⁺ leak and reduced HF progression in WT and RyR2-S2808D^+/+ mice. In contrast, β-adrenergic receptor blockers improved cardiac function in WT but not in RyR2-S2808D^+/+ mice.Thus, chronic PKA hyperphosphorylation of RyR2 results in a diastolic leak that causes cardiac dysfunction. Reversing PKA hyperphosphorylation of RyR2 is an important mechanism underlying the therapeutic action of β-blocker therapy in HF.

Myocardial ischemic disease is the major cause of death worldwide. After myocardial infarction, reperfusion of infracted heart has been an important objective of strategies to improve outcomes. However, cardiac ischemia/reperfusion (I/R) is characterized by inflammation, arrhythmias, cardiomyocyte damage, and, at the cellular level, disturbance in Ca ²⁺ and redox homeostasis. In this study, we sought to determine how acute inflammatory response contributes to reperfusion injury and Ca ²⁺ homeostasis disturbance after acute ischemia. Using a rat model of I/R, we show that circulating levels of TNF-α and cardiac caspase-8 activity were increased within 6 h of reperfusion, leading to myocardial nitric oxide and mitochondrial ROS production. At 1 and 15 d after reperfusion, caspase-8 activation resulted in S-nitrosylation of the RyR2 and depletion of calstabin2 from the RyR2 complex, resulting in diastolic sarcoplasmic reticulum (SR) Ca ²⁺ leak. Pharmacological inhibition of caspase-8 before reperfusion with Q-LETD-OPh or prevention of calstabin2 depletion from the RyR2 complex with the Ca ²⁺ channel stabilizer S107 (“rycal”) inhibited the SR Ca ²⁺ leak, reduced ventricular arrhythmias, infarct size, and left ventricular remodeling after 15 d of reperfusion. TNF-α–induced caspase-8 activation leads to leaky RyR2 channels that contribute to myocardial remodeling after I/R. Thus, early prevention of SR Ca ²⁺ leak trough normalization of RyR2 function is cardioprotective.