Welcome to the Family: Identification of the NAD+ Transporter of Animal Mitochondria as Member of the Solute Carrier Family SLC25

Subcellular compartmentation is a fundamental property of eukaryotic cells. Communication and metabolic and regulatory interconnectivity between organelles require that solutes can be transported across their surrounding membranes. Indeed, in mammals, there are hundreds of genes encoding solute carriers (SLCs) which mediate the selective transport of molecules such as nucleotides, amino acids, and sugars across biological membranes. Research over many years has identified the localization and preferred substrates of a large variety of SLCs. Of particular interest has been the SLC25 family, which includes carriers embedded in the inner membrane of mitochondria to secure the supply of these organelles with major metabolic intermediates and coenzymes. The substrate specificity of many of these carriers has been established in the past. However, the route by which animal mitochondria are supplied with NAD+ had long remained obscure. Only just recently, the existence of a human mitochondrial NAD+ carrier was firmly established. With the realization that SLC25A51 (or MCART1) represents the major mitochondrial NAD+ carrier in mammals, a long-standing mystery in NAD+ biology has been resolved. Here, we summarize the functional importance and structural features of this carrier as well as the key observations leading to its discovery.

Activity-Dependent Gene Expression in Neurons

The function of the nervous system in conveying and processing information necessary to interact with the environment confers unique aspects on how the expression of genes in neurons is regulated. Three salient factors are that (1) neurons are the largest and among the most morphologically complex of all cells, with strict polarity, subcellular compartmentation, and long-distant transport of gene products, signaling molecules, and other materials; (2) information is coded in the temporal firing pattern of membrane depolarization; and (3) neurons must maintain a stable homeostatic level of activation to function so stimuli do not normally drive intracellular signaling to steady state. Each of these factors can require special methods of analysis differing from approaches used in non-neuronal cells. This review considers these three aspects of neuronal gene expression and the current approaches being used to analyze these special features of how the neuronal transcriptome is modulated by action potential firing.

TurboID-mediated proximity labelling of cytoophidium proteome in Drosophila

Proximity-based biotinylation combined with mass spectrometry has emerged as a powerful approach to study protein interaction networks and protein subcellular compartmentation. However, low kinetics and the requirement of toxic chemicals limit the broad utilisation of current proximity labelling methods in living organisms. TurboID, the newly engineered promiscuous ligase, has been reported to label bait proteins effectively in various species. Here, we systematically demonstrated the application of TurboID-mediated biotinylation in a wide range of developmental stages and tissues, and we also verified the feasibility of TurboID-mediated labelling in desired cells via cell-type-specific GAL4 driver in Drosophila. Furthermore, using TurboID-mediated biotinylation coupled with mass spectrometry, we characterized the proximate proteome of the cytoophidium, a newly identified filamentous structure containing the metabolic enzyme CTP synthase (CTPS) in Drosophila. Our study demonstrates a referable tool and resource for research in subcellular compartments of metabolic enzymes in vivo.