Comparative analysis of monoclonal antibodies against prostate-specific membrane antigen (PSMA).

BACKGROUND: Prostate-specific membrane antigen (PSMA), also known as glutamate carboxypeptidase II (GCPII), is generally recognized as a diagnostic and therapeutic cancer antigen and a molecular address for targeted imaging and drug delivery studies. Due to its significance in cancer research, numerous monoclonal antibodies (mAbs) against GCPII have been described and marketed in the past decades. Unfortunately, some of these mAbs are poorly characterized, which might lead to their inappropriate use and misinterpretation of the acquired results. METHODS: We collected the 13 most frequently used mAbs against GCPII and quantitatively characterized their binding to GCPII by enzyme-linked immunosorbent assay (ELISA) and surface plasmon resonance (SPR). Using a peptide library, we mapped epitopes recognized by a given mAb. Finally, we assessed the applicability of these mAbs to routine experimental setups, including Western blotting, immunohistochemistry, and flow cytometry. RESULTS: ELISA and SPR analyses revealed that mAbs J591, J415, D2B, 107-1A4, GCP-05, and 2G7 bind preferentially to GCPII in native form, while mAbs YPSMA-1, YPSMA-2, GCP-02, GCP-04, and 3E6 bind solely to denatured GCPII. mAbs 24.4E6 and 7E11-C5.3 recognize both forms of GCPII. Additionally, we determined that GCP-02 and 3E6 cross-react with mouse GCPII, while GCP-04 recognizes GCPII and GCPIII proteins from both human and mouse. CONCLUSION: This comparative analysis provides the first detailed quantitative characterization of the most commonly used mAbs against GCPII and can serve as a guideline for the scientific community to use them in a proper and efficient way.

GCPII variants, paralogs and orthologs.

Glutamate carboxypeptidase II (GCPII) and its splice variants, paralogs and human homologs represent a family of proteins with diverse tissue distribution, cellular localization and largely unknown function which have been explored only recently. While GCPII itself has been thoroughly studied from different perspectives, as clearly documented in this series of reviews, very little is known about other members of its family, even though they might be biologically relevant. Differential expression of individual GCPII splice variants is associated with tumor progression and prognosis of prostate cancer. The best studied GCPII homolog, GCPIII or NAALADase II, may be a valid pharmaceutical target for itself since it may compensate for a lack of normal GCPII enzymatic activity. Detailed molecular characterization of this family of proteins is thus very important not only with respect to the potential therapeutic use of GCPII inhibitors, but also for better understanding of the biological role of GCPII within as well as outside the nervous system.

Efficient and versatile one-step affinity purification of in vivo biotinylated proteins: expression, characterization and structure analysis of recombinant human glutamate carboxypeptidase II.

Affinity purification is a useful approach for purification of recombinant proteins. Eukaryotic expression systems have become more frequently used at the expense of prokaryotic systems since they afford recombinant eukaryotic proteins with post-translational modifications similar or identical to the native ones. Here, we present a one-step affinity purification set-up suitable for the purification of secreted proteins. The set-up is based on the interaction between biotin and mutated streptavidin. Drosophila Schneider 2 cells are chosen as the expression host, and a biotin acceptor peptide is used as an affinity tag. This tag is biotinylated by Escherichia coli biotin-protein ligase in vivo. We determined that localization of the ligase within the ER led to the most effective in vivo biotinylation of the secreted proteins. We optimized a protocol for large-scale expression and purification of AviTEV-tagged recombinant human glutamate carboxypeptidase II (Avi-GCPII) with milligram yields per liter of culture. We also determined the 3D structure of Avi-GCPII by X-ray crystallography and compared the enzymatic characteristics of the protein to those of its non-tagged variant. These experiments confirmed that AviTEV tag does not affect the biophysical properties of its fused partner. Purification approach, developed here, provides not only a sufficient amount of highly homogenous protein but also specifically and effectively biotinylates a target protein and thus enables its subsequent visualization or immobilization.