Materials and Equipment Required

Before beginning, gather the following materials:

• Recombinant Human CDC34 Protein (SKU: rp377104; absea #PP-10614)
• Reconstitution buffer: sterile, nuclease-free PBS (pH 7.4) or 20 mM Tris-HCl (pH 7.5) supplemented with 10% (v/v) glycerol — consult the specific lot Certificate of Analysis (CoA) for the recommended buffer
• Microcentrifuge tubes (1.5 mL, low-protein-binding)
• Calibrated micropipettes and sterile filtered tips
• Microcentrifuge capable of ≥10,000 × g
• Ice bucket or cooling block
• -80 °C freezer for long-term storage; -20 °C for short-term (≤1 month)
• SDS-PAGE gel system and Coomassie or silver stain (for purity verification)
• Western blot apparatus and anti-CDC34 or anti-His/GST tag antibody (if tag present)
• Ubiquitination assay kit or components: E1 activating enzyme, ubiquitin, ATP/Mg²⁺ reaction buffer (for activity verification)

Note: All manipulations should be performed on ice or at 4 °C to maintain enzyme integrity.

Reconstitution and Storage Procedure

Follow these steps to reconstitute and store Recombinant Human CDC34 Protein safely:

1. Thaw carefully: Remove the vial from -80 °C storage and allow it to thaw on ice for 10–15 minutes. Do not thaw at room temperature or use a water bath above 4 °C.
2. Centrifuge: Briefly spin the vial at 2,000–3,000 × g for 30 seconds to collect lyophilized material or any condensation at the bottom of the tube.
3. Add reconstitution buffer: Using a calibrated micropipette, slowly add the appropriate volume of pre-chilled reconstitution buffer (as specified in the CoA) along the inner wall of the vial to avoid foaming. Gently swirl or pipette mix; do not vortex.
4. Equilibrate: Allow the reconstituted solution to stand on ice for 5–10 minutes with occasional gentle mixing to ensure complete dissolution.
5. Aliquot immediately: Divide the protein into single-use aliquots in low-protein-binding microcentrifuge tubes to avoid repeated freeze-thaw cycles, which reduce E2 enzyme activity.
6. Flash-freeze aliquots: Snap-freeze aliquots in liquid nitrogen or a dry-ice/ethanol bath before transferring to -80 °C for long-term storage.
7. Working stock: For frequent use within one week, store aliquots at 4 °C with 1 mM DTT added as a reducing agent to protect active-site cysteine residues.

Stability: Properly stored aliquots are expected to maintain activity for up to 12 months at -80 °C, though this should be confirmed against your specific lot CoA.

Quality Verification by SDS-PAGE and Western Blot

Before use in downstream assays, verify protein identity, purity, and integrity as follows:

SDS-PAGE Purity Check

1. Prepare a reducing SDS-PAGE sample: mix 1–2 µg of reconstituted CDC34 protein with 4× Laemmli sample buffer containing β-mercaptoethanol; heat at 95 °C for 5 minutes.
2. Load onto a 12–15% polyacrylamide gel alongside a pre-stained molecular weight marker.
3. Run electrophoresis at 100–120 V until the dye front reaches the bottom.
4. Stain with Coomassie Brilliant Blue R-250 or silver stain; destain and image.
5. Expected result: a major band at the predicted molecular weight of CDC34 (approximately 23–26 kDa for the core UBC domain; full-length human CDC34/UBE2R1 is ~28–35 kDa depending on isoform and tag). Verify exact expected MW from the CoA.

Western Blot Confirmation

1. Transfer proteins from the gel to a PVDF or nitrocellulose membrane.
2. Block with 5% non-fat milk in TBST for 1 hour at room temperature.
3. Incubate with primary anti-CDC34 antibody or anti-affinity-tag antibody (e.g., anti-His) per the antibody manufacturer's recommended dilution, overnight at 4 °C.
4. Wash 3× with TBST; incubate with HRP-conjugated secondary antibody for 1 hour at room temperature.
5. Develop with ECL substrate and image.

Acceptance criteria: ≥90% purity by densitometry and a single specific band at the expected molecular weight on Western blot are recommended acceptance criteria before proceeding to functional assays.

Functional Ubiquitination Activity Assay

CDC34 (UBE2R1) functions as an E2 ubiquitin-conjugating enzyme in the ubiquitin–proteasome pathway. Use the following in vitro ubiquitination assay to confirm enzymatic activity:

Reaction Components

• E1 ubiquitin-activating enzyme (commercially available)
• Ubiquitin (human, recombinant)
• Recombinant Human CDC34 Protein (rp377104) — E2 enzyme
• SCF E3 ligase complex or RING-domain E3 ligase (as appropriate for the substrate)
• Substrate protein of interest (e.g., p27^Kip1, IκBα — well-characterized CDC34 substrates)
• Reaction buffer: 50 mM Tris-HCl (pH 7.5), 5 mM MgCl₂, 2 mM ATP, 0.1 mM DTT

Assay Procedure

1. Assemble the reaction on ice in a total volume of 20–30 µL: combine reaction buffer, E1 (50–100 nM), ubiquitin (2–5 µM), CDC34/E2 (200–500 nM), E3 ligase, and substrate at concentrations recommended by respective CoAs or literature.
2. Initiate the reaction by transferring tubes to 37 °C; incubate for 60–120 minutes.
3. Stop the reaction by adding 4× reducing SDS-PAGE sample buffer and heating at 95 °C for 5 minutes.
4. Resolve by SDS-PAGE and detect ubiquitinated substrate by Western blot using an anti-substrate or anti-ubiquitin antibody.
5. Expected result: appearance of high-molecular-weight polyubiquitin ladder bands above the unmodified substrate band, indicating successful E2-mediated ubiquitin transfer.

Positive and negative controls: Include a no-CDC34 control (replace with buffer) and a no-ATP control to confirm reaction specificity. An established positive control with a validated E2 enzyme may also be included.

Note: Specific assay parameters (enzyme concentrations, incubation times) may require optimization for individual experimental systems. Exact working concentrations should be confirmed from the product CoA and relevant literature.

Troubleshooting and Best Practices

Use the table below to diagnose and resolve common issues encountered when working with Recombinant Human CDC34 Protein:

General best practices:

• Always work on ice; minimize time at room temperature.
• Use a reducing agent (DTT or TCEP) in working buffers to preserve the catalytic cysteine of CDC34.
• Quantify protein concentration for each new aliquot using a colorimetric protein assay before use.
• Document lot number and CoA details for every experiment to ensure reproducibility.
• Dispose of protein waste according to your institution's biosafety guidelines.