Highly sensitive detection of E2 activity in ubiquitination using an artificial RING finger

Unique E2-binding specificity of artificial RING fingers in cancer cells

Ubiquitin (Ub)-conjugating enzymes (E2s) are involved in various pathways for Ub transfer and deubiquitinating activities. These enzymes are associated with cancers such as breast cancer which is the second deadliest type of malignancy among women. Here, we revealed the unique E2-binding property and the auto-ubiquitination of artificial RING fingers (ARFs). Circular dichroism spectra showed the characteristic structures of ARFs. The proline, lysine, leucine, threonine and cysteine (PKLTC) sequence of ARF was important for E2-recognition and its mutations induced obvious changes in the E2-binding specificity and the auto-ubiquitination activity of ARF. The ARF mutants were applicable to detection of most of E2 activities. Furthermore, adding the ARF mutant C35A to cancer cells promoted its auto-ubiquitination, leading to the preferential detection of E2 UbcH5b activity. The present work opens up a new avenue for investigating intracellular E2 activities for the fatal diseases.

Artificial RING finger reveals unique auto-ubiquitination with E2 specificity

Ubiquitin (Ub)-conjugating enzymes (E2s) transfer activated Ub from Ub-activating enzymes (E1s) to substrates and are associated with various cancers and neurological disorders. In this study, the unique properties of E2-binding and auto-ubiquitination of artificial RING fingers (ARFs) were demonstrated in ubiquitination assays. Circular dichroism spectra indicated the characteristic structures of ARFs. Point mutations of 31 PKLTC35 in ARF by tryptophan (Trp) resulted in dramatic changes in E2 specificity and the type of Ub chain elongation of mono- and polyubiquitination. The Trp residue was a cue that changed the ubiquitination activity of ARF via E2-binding. Furthermore, the ARF mutants interacted with all 11 E2s and then promoted auto-ubiquitination. Thus, the use of the ARF mutants allowed specific detection of E2 activities during ubiquitination. The present study opens up a new avenue for researching E2 activities related to the fatal diseases.

Solution structure of the zinc finger domain of human RNF144A ubiquitin ligase

RNF144A is involved in protein ubiquitination and functions as an ubiquitin-protein ligase (E3) via its RING finger domain (RNF144A RING). RNF144A is associated with degradation of heat-shock protein family A member 2 (HSPA2), which leads to the suppression of breast cancer cell proliferation. In this study, the solution structure of RNF144A RING was determined using nuclear magnetic resonance. Moreover, using a metallochromic indicator, we spectrophotometrically determined the stoichiometry of zinc ions and elucidated that RNF144A RING binds two zinc atoms. This structural analysis provided the position and range of the active site of RNF144A RING at the atomic level, which contributes to the creation of artificial RING fingers having the specific ubiquitin-conjugating enzyme (E2)-binding capability.

Zinc finger domain of the human DTX protein adopts a unique RING fold

The Deltex (DTX) family is involved in ubiquitination and acts as Notch signaling modifiers for controlling cell fate determination. DTX promotes the development of the ubiquitin chain via its RING finger (DTX_RING). In this study, the solution structure of DTX_RING was determined using nuclear magnetic resonance (NMR). Moreover, by experiments with a metallochromic indicator, we spectrophotometrically estimated the stoichiometry of zinc ions and found that DTX_RING possesses zinc-binding capabilities. The Simple Modular Architecture Research Tool database predicted the structure of DTX_RING as a typical RING finger. However, the actual DTX_RING structure adopts a novel RING fold with a unique topology distinct from other RING fingers. We unveiled the position and the range of the DTX_RING active site at the atomic level. Artificial RING fingers (ARFs) are made by grafting active sites of the RING fingers onto cross-brace structure motifs. Therefore, the present structural analysis could be useful for designing a novel ARF.

Unique RING finger structure from the human HRD1 protein

Artificial RING fingers (ARFs) are created by transplanting active sites of RING fingers onto cross-brace structures. Human hydroxymethylglutaryl-coenzyme A reductase degradation protein 1 (HRD1) is involved in the degradation of the endoplasmic reticulum (ER) proteins. HRD1 possesses the RING finger domain (HRD1_RING) that functions as a ubiquitin-ligating (E3) enzyme. Herein, we determined the solution structure of HRD1_RING using nuclear magnetic resonance (NMR). Moreover, using a metallochromic indicator, we determined the stoichiometry of zinc ions spectrophotometrically and found that HRD1_RING binds to two zinc atoms. The Simple Modular Architecture Research Tool database predicted the structure of HRD1_RING as a typical RING finger. However, it was found that the actual structure of HRD1_RING adopts an atypical RING-H₂ type RING fold. This structural analysis unveiled the position and range of the active site of HRD1_RING that contribute to its specific ubiquitin-conjugating enzyme (E2)-binding capability.

Unique auto-ubiquitination activities of artificial RING fingers in cancer cells

Ubiquitin-conjugating (E2) enzymes in protein ubiquitination are associated with various diseases. An artificial RING finger (ARF) is a useful tool, and E2 activities are conveniently estimated based on ARF reactivities. To extend the use of ARF in cells, we constructed a TAT-ARF using a cell-penetrating trans-activator protein (TAT) peptide. An in vitro ubiquitination assay without substrates showed auto-ubiquitination of TAT-ARF via its TAT region. TAT-ARF was translocated into MCF7 breast cancer cells, and then TAT-ARF ubiquitinated itself via its ARF. Experiments using confocal laser-scanning microscopy revealed that FAM-labeled TAT-ARF was readily internalized in cells and it remained encapsulated in vesicles. The Cell Counting Kit-8 assay indicated that the TAT-ARF uptake occurred without cytotoxicity in MCF7 cells at concentrations below 5.0 μM. By taking advantage of TAT-ARF, we, for the first time, succeeded in detecting E2 activities in cells. Thus, the present work opens up new avenues in the investigation of protein ubiquitination.

Concise machinery for monitoring ubiquitination activities using novel artificial RING fingers

Protein ubiquitination is involved in many cellular processes, such as protein degradation, DNA repair, and signal transduction pathways. Ubiquitin-conjugating (E2) enzymes of the ubiquitination pathway are associated with various cancers, such as leukemia, lung cancer, and gastric cancer. However, to date, detection of E2 activities is not practicable for capturing the pathological conditions of cancers due to complications related to the enzymatic cascade reaction. To overcome this hurdle, we have recently investigated a novel strategy for measuring E2 activities. Artificial RING fingers (ARFs) were developed to conveniently detect E2 activities during the ubiquitination reaction. ARFs were created by grafting the active sites of ubiquitin-ligating (E3) enzymes onto amino acid sequences with 38 residues. The grafting design downsized E3s to small molecules (ARFs). Such an ARF is a multifunctional molecule that possesses specific E2-binding capabilities and ubiquitinates itself without a substrate. In this review, we discuss the major findings from recent investigations on a new molecular design for ARFs and their simplified detection system for E2 activities. The use of the ARF allowed us to monitor E2 activities using acute promyelocytic leukemia (APL)-derived cells following treatment with the anticancer drug bortezomib. The molecular design of ARFs is extremely simple and convenient, and thus, may be a powerful tool for protein engineering. The ARF methodology may reveal a new screening method of E2s that will contribute to diagnostic techniques for cancers.

Solution structure of the PHD finger from the human KIAA1045 protein

Cross-brace structural motifs are required as a scaffold to design artificial RING fingers (ARFs) that function as ubiquitin ligase (E3) in ubiquitination and have specific ubiquitin-conjugating enzyme (E2)-binding capabilities. The Simple Modular Architecture Research Tool database predicted the amino acid sequence 131-190 (KIAA1045ZF) of the human KIAA1045 protein as an unidentified structural region. Herein, the stoichiometry of zinc ions estimated spectrophotometrically by the metallochromic indicator revealed that the KIAA1045ZF motif binds to two zinc atoms. The structure of the KIAA1045ZF motif bound to the zinc atoms was elucidated at the atomic level by nuclear magnetic resonance. The actual structure of the KIAA1045ZF motif adopts a C₄ HC₃ -type PHD fold belonging to the cross-brace structural family. Therefore, the utilization of the KIAA1045ZF motif as a scaffold may lead to the creation of a novel ARF.

The zinc finger domain of RING finger protein 141 reveals a unique RING fold

Human RING finger protein 141 (RFP141) is a germ cell-specific transcription factor during spermatogenesis. We synthesized a compact construct encoding the C-terminal zinc finger of RFP141 (RFP141C peptide). Herein we determined the solution structure of the RFP141C peptide by nuclear magnetic resonance (NMR). Moreover, NMR data and the chemical modification of cysteine residues demonstrated that the RFP141C peptide binds to two zinc atoms in a cross-brace arrangement. The Simple Modular Architecture Research Tool database predicted the structure of RFP141C as a RING finger. However, the actual structure of the RFP141C peptide adopts an atypical compact C₃ HC₄ -type RING fold. The position and range of the helical active site of the RFP141C structure were elucidated at the atomic level. Therefore, structural analysis may allow RFP141C to be used for designing an artificial RING finger possessing specific ubiquitin-conjugating enzyme (E2)-binding capabilities.