Chinese hamster ovary (CHO-K1) cells were cultured in CDM4CHO medium with
l-glutamine (Cat# SH30557.02, Hyclone/Cytiva, Marlborough, MA, USA). Cells (1 × 10
5) were mixed with 2 μg of 100 μg/ml of murine NEU3 expression plasmid (MR223297; Origene, Rockville, MD, USA) in 100 μl PBS (GE Lifesciences, Marlborough, MA, USA) and were transfected by electroporation using a 4D-Nucleofector System (Lonza, Walkersville, MD, USA) following the manufacturer’s protocol. Before use, the plasmid was sequenced for verification as previously described [
25,
27‐
29]. The transfected cells were kept at room temperature for 15 min for recovery, after which the CHO-K1 cells were cultured in 25 ml CDM4CHO medium in a humidified incubator at 37 °C with 5% CO
2. After 24 h, 400 μg/ml of G418 (345812; Calbiochem EMD, San Diego, CA) was added to select for transfected cells. After 10 days, the cells were collected and lysed, and c-Myc–tagged recombinant murine NEU3 was purified using a Myc-Trap agarose kit (ytak-20; Chromotek, Hauppauge, NY, USA) following the manufacturer’s protocol. Recombinant protein was checked for protein concentration by OD 260/280/320 using a Take3 micro-volume plate with a SynergyMX plate reader (BioTek, Winooski, VT, USA). NEU3 is 51 kDa, and was further purified by centrifugation at 10,000×
g for 5 min at 4 °C through an Amicon Ultra 0.5 ml 100 kDa cutoff spin filter (Millipore, Billerica, MA, USA). The NEU3 was analyzed for size and purity by PAGE on, and Coomassie staining of, 4–20% Tris/glycine gels (Bio-Rad, Hercules, CA, USA), as described previously [
25,
27‐
29]. The NEU3 was stored in 50 μl of 10% glycerol, 100 mM glycine, and 25 mM Tris–HCl, pH 7.3, at 4 °C.