Molecular mechanisms regulating the form and function of atypical calcium sensor proteins.

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Cells with a discrete nucleus use calcium ions (Ca2+) to transmit signals that are an essential part of their lifecycle. These signals include both normal and danger responses such as reproduction, cell stress and death. All these cells have evolved a toolkit comprised of protein molecules that are tailored to sense Ca2+ changes in specific cellular compartments. Many toolkit components rely on vast differences in Ca2+ levels to mediate precise cell signals. For example, stromal interaction molecules (STIMs) are proteins located inside the cell on the endoplasmic reticulum (ER) membrane, which encloses a specialized compartment containing high Ca2+ levels. STIMs respond to the depletion of ER Ca2+ by interacting with Orai channel proteins located on the outside plasma membrane of the cell. This interaction causes Orai channels to open, permitting Ca2+ to move into the cell. Mitochondria take up this increased Ca2+ to influence intracellular Ca2+ signals and regulate the production of ATP, the energy currency of the cell. However, prolonged increases in mitochondrial Ca2+ can be catastrophic as cell death pathways are activated. Thus, mitochondria have evolved the use of the leucine zipper EF-hand containing transmembrane protein-1 (LETM1) protein to precisely control mitochondrial Ca2+ levels. Both STIMs and LETM1 contain a protein apparatus known as the EF-hand motif, which typically undergoes structural changes after interacting with Ca2+. These conformational changes trigger downstream protein-protein interactions and signal propagation. Remarkably, the STIM and LETM1 Ca2+ binding EF-hands appear to be atypically unpaired. Further, the manner that these EF-hands function based on the atomic compositions and the effects of environment-specific chemical modifications remain poorly understood. Thus, this research program aims to expose how single Ca2+ binding EF-hands have been integrated into cellular Ca2+ sensing components, how chemical modifications caused by cell stress effect sensitivities to Ca2+ and how the EF-hand structures and protein interaction targets change with and without Ca2+. Our research will provide new insights into key features that establish the specific Ca2+ sensory functions of STIMs and LETM1. Further, we will reveal the role of naturally occurring chemical modifications in modulating STIM function and provide the first atomic-level insights into how LETM1 EF-hand structure is related to function. Collectively, this knowledge will provide a framework for the development of new research tools, engineered to sense changes in Ca2+, cell stresses and control cellular responses, as well as drive future research hypothesis aimed at fully understanding the mechanistic basis for the ubiquitous STIM and LETM1 functions. The program will benefit Canada by providing multidisciplinary training for numerous diverse individuals, which will help secure future careers and innovations in the natural sciences.