Analysis of membrane receptor of aralin, a cancer-selective cytotoxic protein from aralin elata

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Aralin from Aralia elatais a new type II ribosome inactivating protein (RIP). Its A-chain exhibits RNA N-glycosidase activity to inactivate the ribosome and inhibit protein synthesis, while B-chain is the Gal and its derivatives-specific lectin. Aralin preferentially induces apoptosis in cancer cells compared with normal cells. To identify the potent aralin receptor, we previously analyzed the membrane proteins by far Western blotting with anti-aralin antibody, and LC/MS. The obtained data suggested that aralin receptor is the 110-kDa high density lipoprotein binding protein (HDLBP), which is processed from 150-kDa HDLBP and existing in lipid raft as an active HDL receptor. The expression levels of 110-kDa HDLBP of various cancer cells were higher than those of normal cells. Furthermore, we established 110-kDa HDLBP-knockdown HeLa cells using miRNAs. The sensitivity of these cells against aralin was robustly reduced. In contrast, 110-kDa HDLBP-over-expressing cells were not obtained by only forced expression of 150-kDa HDLBP. Expectedly, sensitivity of these cells against aralin was comparable to the control cells. Thus, these results indicate that the processed 110-kDa HDLBP is the authentic receptor and its expression level in the lipid raft determines the sensitivity toward aralin. HDLBP processing analysis from 150-kDa to 110-kDa active form using N- and C-terminal-tagged 150-kDa HDLBP indicated that the N-terminal region could be removed. Currently, we were pursuing the identification of the N-terminal cutting site and possible processing enzymes. In addition, we are exploring the processing mechanism of the HDLBP affecting variety of cancer cells.