LAUSR

Every breath of life and every beat of the heart relies on the faithful translation of fast voltage signals into a dynamic influx of Ca ions that initiates the contraction of skeletal and cardiac muscle. This process, known as excitation– contraction coupling, requires the precise geometric arrangement and functional coordination of two large ion channel macromolecular complexes that reside in two opposing membrane compartments which are separated by a narrow cleft-the L-type voltage-gated Ca channels (CaV1) in the traverse-tubular (t-tubule) membrane and the ryanodine receptor (RyR) in the sarcoplasmic reticulum (SR). In the heart, this compartment is known as a dyad based on its bipartite organization (Fig. 1). The propagation of the cardiac action potential into the t-tubule results in the activation of dyadic L-type channels (CaV1.2) that convey a local influx of Ca into the cleft. The freely diffusing Ca, in turn, binds RyR2 across the dyadic cleft to elicit a much larger release of Ca from intracellular stores (1). In addition to the two ion channel behemoths, a wide range of Ca-effector proteins such as calmodulin-dependent kinases and calcineurin also reside in this space and couple changes in Ca signals to various cellular functions, rendering the dyadic junction a privileged Ca-signaling domain (1). In the skeletal muscle, the analogous subcellular compartment is known as a triad owing to its tripartite arrangement (Fig. 1). In the triad, voltage-dependent activation of the CaV1.1 L-type channel is conformationally coupled to RyR1, thus obviating the need for Ca ions in evoking Ca release from the intracellular store (2). Beyond striated muscle, close apposition of the endoplasmic reticulum and the plasma membrane (ER–PM junctions) is found in many cell types including neurons, where they play a vital role in Ca homeostasis and signaling and lipid exchange (3, 4). The function and formation of ER–PM junctions can be static or dynamic, involving a rich repertoire of proteins. For example, depletion of intracellular Ca stores causes the coaccumulation of stromal interaction molecule and Orai channels in ER–PM junctions (5). Ensuing localized Ca entry through Orai replenishes the intracellular stores (5). By comparison, dyadic and triadic junctions are preassembled. In this broader context, the junctophilin family of proteins (Jph1 to Jph4) have been shown to play an essential role in both the formation of ER–PM junctions in muscle and neurons as well as to serve as a scaffold that recruits relevant ion channels to this specialized Ca-signaling compartment (6). In PNAS, the paper by Yang et al. (7) presents high-resolution atomic structures of a key Jph domain alone and associated with an isolated cytosolic segment of the CaV1.1 channel. This study opens new frontiers in understanding the structure and mechanisms underlying the formation of ER–PM junctions and the recruitment of ion channels to this subcellular domain. The junctophilin family of proteins were identified in 2000 as an important component of junctional membrane complexes by Takeshima et al. (8). Four different subtypes of Jph have been identified, with Jph1 primarily expressed in skeletal muscle, Jph2 in cardiac and skeletal muscle, and Jph3 and Jph4 in neurons (see reviews in refs. 6 and 9). In terms of molecular architecture, the Jphs are composed of eight membrane occupation and recognition nexus (MORN) domains, a long α-helical region, thought to be a spacer between the ER and PM, a long divergent region, and a transmembrane domain. The first six MORN domains are linked to the remaining two via a joining region that is largely disordered. The MORN domains are thought to associate with the PM through interactions with phosphoinsoitol-containing phospholipids, palmitoylation, or association with adapter proteins, while the transmembrane domain spans the ER (6, 9). The α-helical domain has been proposed to function as a molecular spacer that determines the size of the cleft. Beyond its structural role, Jph1 to Jph4 have emerged as versatile modulators of various ion channels, recruiting and fine-tuning the function of RyR1 to RyR3 (10–12), CaV1/2 (11, 13), and KCNQ channels (10). In skeletal muscle, Jph1 recruits CaV1.1 to triads through a physical interaction with the channel carboxy terminus (13) and serves as one of the minimal requisite components necessary for reconstituting voltagedependent excitation–contraction coupling (the others being the CaV1.1 β1a subunit and the adaptor protein stac3) (14). In the heart, the interaction of Jph2 with RyR2 stabilizes channel function and prevents diastolic Ca leak (12). In the brain, Jph3/4 recruits CaV1 and CaV2 channels to ER–PM junctions and can selectively modify the inactivation kinetics of CaV2 channels (11). Not surprisingly, constitutive knockout of Jph1 and Jph2 is lethal, while knockout of Jph3 and Jph4 results in deficits in motor coordination, learning, and memory (6). In the heart, acute loss of Jph2 has been shown to result in heart failure and increased mortality, presumably due to deficits in t-tubule maturation and altered Ca homeostasis (12). By contrast, Jph2 overexpression promotes t-tubule formation and is protective against heart failure in mice (15). Human mutations in