Schematic of the photocatalytic proximity labeling approach: A photocatalyst is attached to four-stranded G-quadruplex DNA; blue light produces short-lived singlet oxygen, so only the proteins closest to the structure are tagged.
Image Credit: © Ahmed Mostafa Abdelhady and Kazumitsu Onizuka, Tohoku University.
Scientific Frontline: Extended "At a Glance" Summary: Photocatalytic Proximity Labeling of G-Quadruplex DNA
The Core Concept: Photocatalytic proximity labeling is a novel biochemical technique that attaches a light-reactive catalyst to four-stranded G-quadruplex DNA to precisely identify nearby interacting proteins.
Key Distinction/Mechanism: Unlike conventional bait-and-capture methods that miss transient binders, or chemical probes that block interaction sites, this approach uses blue light to generate short-lived singlet oxygen, chemically tagging only the proteins situated within a few nanometers of the DNA structure.
Major Frameworks/Components:
- G-Quadruplex (G4) DNA: Compact, four-stranded DNA bundles that form in guanine-rich genomic regions, such as telomeres and gene control centers.
- Photocatalytic Tagging: A process utilizing thirty seconds of blue light to produce reactive singlet oxygen for localized, highly specific chemical tagging.
- Mass Spectrometry: The analytical tool used to identify the newly tagged candidate binding proteins.
- Hexokinase-1 (HK1): A canonical metabolic enzyme involved in glycolysis that this method unexpectedly revealed to be a tight G4-binding protein.
Branch of Science: Molecular Biology, Biochemistry, and Genomics.
Future Application: The technique serves as a general platform for mapping protein interactions across other complex nucleic acid structures, potentially illuminating new therapeutic targets for cancer and metabolic diseases.
Why It Matters: This tool overcomes the limitations of previous assays to provide a much clearer picture of gene regulation. Its discovery of HK1 binding directly to DNA suggests a much tighter, more direct link between cellular metabolism and genetic control than researchers previously theorized.
DNA can be more than just a double helix. When four strands are folded into a compact bundle, they form a G-quadruplex (G4). These bundles form in guanine-rich regions across the genome, including telomeres at the ends of chromosomes and the control regions of genes, where the proteins that dock onto them help determine which genes are switched on. G4 does not act uniformly everywhere. Its function depends on where it forms and which proteins bind to it.
This is why understanding the interactions surrounding G4 is so important.
Biochemical assays provide more information about the intricacies of these interactions. These assays assess the way DNA and proteins interact during processes such as DNA replication, transcription, and repair. However, previous assays have not been able to capture the complete picture.
A team led by Kazumitsu Onizuka and Shinichi Sato of Tohoku University, along with Takanori Oyoshi of Shizuoka University, has developed a photocatalytic proximity labeling method that overcomes previous pitfalls and captures partners that earlier methods missed. Furthermore, the technique revealed hexokinase-1 to be an unexpected player in these interactions.
"The problem is that identifying the proteins that bind G4 is not at all straightforward," explains Sato. "Conventional bait-and-capture methods miss weak or transient binders. Newer chemical probes also have downsides, as they bind into the quadruplex themselves and block some of the proteins they are meant to catch."
The researchers tested their new method by attaching a photocatalyst (which reacts to light) directly to a human telomere G4 and adding a small tagging reagent. Thirty seconds of blue light generated singlet oxygen, a reactive form of oxygen that diffuses only a few nanometers, ensuring that only proteins sitting immediately beside the structure received a chemical tag that could later be identified by mass spectrometry.
The new method flagged more than a thousand candidate binders, but the one that ranked first was unexpected. It was hexokinase-1 (HK1)—an enzyme that carries out the first step of glucose metabolism and normally works at the outer membrane of mitochondria.
"This finding was a genuine surprise," remarks Onizuka. "HK1 is a textbook metabolic enzyme usually associated with glycolysis, and as far as we know, no direct interaction with a folded nucleic acid had been reported for it before."
The binding was confirmed with purified protein by two independent assays. Electrophoretic mobility shift assays showed that HK1 forms a complex with G4 DNA, but not with double-stranded DNA, and microscale thermophoresis confirmed that the interaction is tight. This exciting finding suggests that metabolism and gene regulation, which are usually studied separately, may be more directly connected than previously assumed.
While the biological relationship between HK1 and G4 is still unclear, the team is excited to tackle this question next, as it may help researchers better understand cancer and metabolic diseases.
"The method we developed is not just a one-off," says Ahmed Mostafa Abdelhady, who worked extensively on this project when he was a doctoral student at Tohoku University. "It's more like a general platform that could work for other nucleic acid structures. We are excited to test other candidates as well."
Published in journal: Communications Chemistry
Title: Photocatalytic proximity labeling for the identification of G-quadruplex DNA-interacting proteins
Authors: Ahmed Mostafa Abdelhady, Shinichi Sato, Tatsuki Masuzawa, Keishi Deguchi, Mizuki Oba, Takemaru Sato, Nodoka Mase, Toshifumi Yamanaka, Jamila Abbas Osman, Maho Kato, Keita Nakane, Zhengyi Liu, Kazuki Kuwahara, Satoru Nagatoishi, Kouhei Tsumoto, Fumi Nagatsugi, Takanori Oyoshi, and Kazumitsu Onizuka
Source/Credit: Tohoku University
Edited by: Scientific Frontline
Reference Number: mbio100626_01