[Forum] Professor Donald L. Gill, Department of Cellular and Molecular Physiology, Penn State University College of Medicine

“ Calcium Signaling through ER-PM Junctions ”

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October 20, 2017 16:00 - 17:00


BSI Central Building 1F Seminar Room


Ion channels transduce primary signals through gating mechanisms of extraordinary molecular precision. The widely expressed Orai1 plasma membrane (PM) Ca2+ entry channel is gated by the endoplasmic reticulum (ER) Ca2+-sensing STIM proteins through a unique intermembrane con¬formational coupling mechanism. Triggered by ER Ca2+ store release, STIM proteins migrate into ER-PM junctions where they tether and activate PM Orai1 channels. Orai1-mediated “store-operated” Ca2+ signals control gene expression, growth, secretory, and motile responses universally among cells and are implicated in a spectrum of immunological, muscular and inflammatory disease states. Despite intense study, the molecular nature of the STIM-Orai1 coupling interface and the mechanism of Orai1 channel activation remain obscure. Our recent studies reveal that the STIM-Orai Activating Region (SOAR) of the STIM1 molecule contains a crucial phenylalanine residue. In STIM2, this residue is instead a leucine and accounts for the lower efficacy of STIM2. Mutated to histidine, the STIM1 molecule loses its ability to interact with Orai1 and gate its channel activity. The SOAR unit is STIM1 is a symmet¬rical dimer, yet only one of the two active sites is sufficient to fully activate the Orai1 channel. We propose a “unimolec¬ular” coupling model whereby only one SOAR unit of the STIM1 dimer binds to an Orai1 channel subunit. The other unit in the dimer is able to interact with another Orai1 channel unit on a separate channel molecule. In this way, STIM1 can cross-link between Orai1 channels and cause them to cluster, with important consequences for the generation of Ca2+ signals. In our new studies, we define a discrete segment in the Orai1 protein that creates a critical nexus between the peripheral C-terminal STIM1-binding site and the inner core helices surrounding the central N-terminal pore. The nexus comprises a flexible “hinge” and hydro¬phobic “hinge plate” attaching it to the channel body. Mutation of four critical amino acids in the nexus transforms the Orai1 channel into a persistently open state, virtually identical to the STIM1-activated state. Yet, the mutant is independent of STIM1 and still fully active after removal of the entire STIM1-binding site adjacent to the nexus. The results militate against a complex and widely-held two-site gating model involving direct STIM1 binding to the N-terminal pore-forming helix to open the channel. Instead, we conclude that binding of STIM1 to gate the channel is restricted to the peripheral C-terminal extension helix which remotely controls gating of the Orai1 pore through a simple conforma¬tion¬al switch mediated by the nexus and trans¬duced through the core helices to rearrange the pore mouth and open the channel. Our work reveals that both small interacting regions in the STIM1 and Orai1 molecules are sufficient to effectively mediate gating and clustering of the Orai1 channels.

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Katsuhiko Mikoshiba [Katsuhiko Mikoshiba, Developmental Neurobiology ]
Name: Katsuhiko Mikoshiba