Glutamate is essential for proper brain function as it is our nervous systems principal excitatory neurotransmitter, a signal that stimulates nerve cells to send messages to other cells.
Glutamate activates ionotropic glutamate receptors (iGluRs), which are linked to several neurological diseases in cases when they are improperly regulated. iGluRs are transmembrane channels that allow calcium, as well as other cations, into the post synaptic neuron upon binding of glutamate or other agonists.Interestingly, iGluR homologs in plants also mediate calcium signaling upon glutamate activation and were accordingly named glutamate receptor-like channels (GLRs). Cell signaling is critical for plant survival to mediate rapid response to growth, defense, and other environmental cues.
GLRs are found in all plants and vital for their health, hardiness, and adaptation for growth and survival in unfavorable conditions, such as drought, nutrient poor soil, temperature extremes, pathogens, and predators. Plant research is important with vast applications. Firstly, crops are our primary source of nutrition. In addition, plants are used as sources of drugs that we employ for treating diseases. Some examples of plant-derived neuroactive compounds include caffeine in coffee beans, nicotine in tobacco, and opium from poppy plants. In short, optimizing plant growth is beneficial to maintaining our own survival and potentially achievable by understanding GLRs role in plant health and hardiness. Despite their importance for cell signaling and implication in plant defense and regeneration, the structural basis underlying the function of these channels remains ambiguous, representing a critical barrier to our understanding of GLR function.
To address this problem, I dedicated my thesis work to study the structure of GLRs and gain insight into their function. There are 20 GLRs in the model plant organism, Arabidopsis thaliana, classified into 3 different clades (AtGLR1-3). To narrow down which AtGLRs to focus our structural studies on, we investigated clade 3 representatives, as many of these GLR3s have been extensively studied in different plant species, especially crops. For example, studying AtGLR3.4 could provide useful information to how the homolog in rice, OsGLR3.4, contributes to growth and production in rice. Studying AtGLR3.4’s structure may elucidate how agonistic or antagonistic targets bind and gate the channel, potentially revealing “druggable” targets to alter plant response for defense and regeneration.
Without any structural information available for GLRs, I started my studies by first focusing on their mammalian homologs, iGluRs. I first designed multiple constructs for heterologous expression and purification from cell culture (for example HEK293S GnTI- cells). Then, I optimized protein extraction and purification to obtain pure protein samples. Purified proteins were then subjected to cryo-electron microscopy (cryo-EM) which eventually allowed us to solve the structure of AtGLR3.4, the first full-length GLR structure.
AtGLR3.4’s structure revealed similarities to structures of its mammalian homologs, iGluRs. In comparison to iGluRs, our GLR structure also showed tetrameric subunit assembly, with a three-layer architecture that includes the ligand binding domain (LBD) in the middle, sandwiched between the extracellular amino terminal domain (ATD) at the top and the transmembrane domain (TMD) at the bottom. In contrast to the majority of iGluR structures, however, AtGLR3.4 displayed unique symmetry and domain arrangement with the non-swapped extracellular ATD and LBD domains. We also provided further evidence supporting ligand binding promiscuity that was previously revealed in isolated LBD crystal structures from other AtGLR3s. Surprisingly, we found endogenous glutathione bound to the ATDs and demonstrated its contribution to channel activity.
It is important to fill the gaps in knowledge about GLR structure to understand how these channels are activated and gated. In doing so, we will learn more about iGluRs as well as better understand plant defense and growth, which has the potential to enhance crop production for food security and our overall survival.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/tmh6-xs85 |
| Date | January 2023 |
| Creators | Green, Marriah Noel |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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