Initial characterization of the apoptosis-inducing receptor for natural human anti-neuroblastoma IgM

Background. Human neuroblastoma (NB) cells contain a 260 kDa surface antigen (NBp260), which serves as receptor for natural human IgM antibodies (anti-NB IgM). Upon binding to NB-p260, these antibodies induce apoptosis in human NB cells. Procedure and Results. In this study, we purified NB-p260 to homogeneity from human LA-N-1 NB cells by sequential ion exchange chromatography followed by preparative SDS gel electrophoresis. Purified NB-p260 exhibited rapid autodegradation despite the presence of various protease inhibitors. The autodegradation process precluded extensive N-terminal sequencing. However, from repeat N-terminal sequence analysis, a consensus sequence of seven amino acid residues emerged that exhibited significant homology to the subunit c of the human mitochondrial ATP synthase, a hydrophobic membrane protein of 7.6 kDa. Western blot analyses demonstrated that purified NB-p260 was recognized by polyclonal antibodies raised against both subunit c-containing storage bodies and a synthetic peptide consisting of amino acid residues 32-45 of subunit c. In addition to peptide sequences related to subunit c, NB-p260 also contained epitopes related to the human heat shock protein HSP90. In Western blots, a monoclonal anti-HSP90 antibody reacted with purified NB-p260 as well as with a predominant protein fragment of approximately 90 kDa that appeared during the process of NB-p260 autodegradation. The anti-HSP90 antibody was also capable of binding to the surface of LA-N-1 cells and inhibiting the binding of human anti-NB IgM in a dose-dependent manner. Conclusions. Collectively, our data suggest that NBp260, the apoptosis-mediating receptor for natural human anti-NB IgM, represents a novel surface protein of human NB cells containing polypeptide sequences related to the subunit c of the mitochondrial ATP synthase and the heat shock protein HSP90.