Structural mechanisms for VMAT2 inhibition by tetrabenazine.

The vesicular monoamine transporter 2 (VMAT2) is a proton-dependent antiporter responsible for loading monoamine neurotransmitters into synaptic vesicles. Dysregulation of VMAT2 can lead to several neuropsychiatric disorders including Parkinson's disease and schizophrenia. Furthermore, drugs such as amphetamine and MDMA are known to act on VMAT2, exemplifying its role in the mechanisms of actions for drugs of abuse. Despite VMAT2's importance, there remains a critical lack of mechanistic understanding, largely driven by a lack of structural information. Here we report a 3.1 Å resolution cryo-EM structure of VMAT2 complexed with tetrabenazine (TBZ), a non-competitive inhibitor used in the treatment of Huntington's chorea. We find TBZ interacts with residues in a central binding site, locking VMAT2 in an occluded conformation and providing a mechanistic basis for non-competitive inhibition. We further identify residues critical for cytosolic and lumenal gating, including a cluster of hydrophobic residues which are involved in a lumenal gating strategy. Our structure also highlights three distinct polar networks that may determine VMAT2 conformational dynamics and play a role in proton transduction. The structure elucidates mechanisms of VMAT2 inhibition and transport, providing insights into VMAT2 architecture, function, and the design of small-molecule therapeutics.

Structural mechanisms for VMAT2 inhibition by tetrabenazine.

The vesicular monoamine transporter 2 (VMAT2) is a proton-dependent antiporter responsible for loading monoamine neurotransmitters into synaptic vesicles. Dysregulation of VMAT2 can lead to several neuropsychiatric disorders including Parkinson's disease and schizophrenia. Furthermore, drugs such as amphetamine and MDMA are known to act on VMAT2, exemplifying its role in the mechanisms of actions for drugs of abuse. Despite VMAT2's importance, there remains a critical lack of mechanistic understanding, largely driven by a lack of structural information. Here, we report a 3.1 Å resolution cryo-electron microscopy (cryo-EM) structure of VMAT2 complexed with tetrabenazine (TBZ), a non-competitive inhibitor used in the treatment of Huntington's chorea. We find TBZ interacts with residues in a central binding site, locking VMAT2 in an occluded conformation and providing a mechanistic basis for non-competitive inhibition. We further identify residues critical for cytosolic and lumenal gating, including a cluster of hydrophobic residues which are involved in a lumenal gating strategy. Our structure also highlights three distinct polar networks that may determine VMAT2 conformational dynamics and play a role in proton transduction. The structure elucidates mechanisms of VMAT2 inhibition and transport, providing insights into VMAT2 architecture, function, and the design of small-molecule therapeutics.

Dwarf open reading frame (DWORF) is a direct activator of the sarcoplasmic reticulum calcium pump SERCA.

The sarco-plasmic reticulum calcium pump (SERCA) plays a critical role in the contraction-relaxation cycle of muscle. In cardiac muscle, SERCA is regulated by the inhibitor phospholamban. A new regulator, dwarf open reading frame (DWORF), has been reported to displace phospholamban from SERCA. Here, we show that DWORF is a direct activator of SERCA, increasing its turnover rate in the absence of phospholamban. Measurement of in-cell calcium dynamics supports this observation and demonstrates that DWORF increases SERCA-dependent calcium reuptake. These functional observations reveal opposing effects of DWORF activation and phospholamban inhibition of SERCA. To gain mechanistic insight into SERCA activation, fluorescence resonance energy transfer experiments revealed that DWORF has a higher affinity for SERCA in the presence of calcium. Molecular modeling and molecular dynamics simulations provide a model for DWORF activation of SERCA, where DWORF modulates the membrane bilayer and stabilizes the conformations of SERCA that predominate during elevated cytosolic calcium.

Intrinsically disordered HAX-1 regulates Ca2+ cycling by interacting with lipid membranes and the phospholamban cytoplasmic region.

Hematopoietic-substrate-1 associated protein X-1 (HAX-1) is a 279 amino acid protein expressed ubiquitously. In cardiac muscle, HAX-1 was found to modulate the sarcoendoplasmic reticulum calcium ATPase (SERCA) by shifting its apparent Ca2+ affinity (pCa). It has been hypothesized that HAX-1 binds phospholamban (PLN), enhancing its inhibitory function on SERCA. HAX-1 effects are reversed by cAMP-dependent protein kinase A that phosphorylates PLN at Ser16. To date, the molecular mechanisms for HAX-1 regulation of the SERCA/PLN complex are still unknown. Using enzymatic, in cell assays, circular dichroism, and NMR spectroscopy, we found that in the absence of a binding partner HAX-1 is essentially disordered and adopts a partial secondary structure upon interaction with lipid membranes. Also, HAX-1 interacts with the cytoplasmic region of monomeric and pentameric PLN as detected by NMR and in cell FRET assays, respectively. We propose that the regulation of the SERCA/PLN complex by HAX-1 is mediated by its interactions with lipid membranes, adding another layer of control in Ca2+ homeostatic balance in the heart muscle.

Newly Discovered Micropeptide Regulators of SERCA Form Oligomers but Bind to the Pump as Monomers.

The recently-discovered single-span transmembrane proteins endoregulin (ELN), dwarf open reading frame (DWORF), myoregulin (MLN), and another-regulin (ALN) are reported to bind to the SERCA calcium pump in a manner similar to that of known regulators of SERCA activity, phospholamban (PLB) and sarcolipin (SLN). To determine how micropeptide assembly into oligomers affects the availability of the micropeptide to bind to SERCA in a regulatory complex, we used co-immunoprecipitation and fluorescence resonance energy transfer (FRET) to quantify micropeptide oligomerization and SERCA-binding. Micropeptides formed avid homo-oligomers with high-order stoichiometry (n > 2 protomers per homo-oligomer), but it was the monomeric form of all micropeptides that interacted with SERCA. In view of these two alternative binding interactions, we evaluated the possibility that oligomerization occurs at the expense of SERCA-binding. However, even the most avidly oligomeric micropeptide species still showed robust FRET with SERCA, and there was a surprising positive correlation between oligomerization affinity and SERCA-binding. This comparison of micropeptide family members suggests that the same structural determinants that support oligomerization are also important for binding to SERCA. Moreover, the unique oligomerization/SERCA-binding profile of DWORF is in harmony with its distinct role as a PLB-competing SERCA activator, in contrast to the inhibitory function of the other SERCA-binding micropeptides.