Monoclonal antibodies specific for human tumor‐associated antigen 90K/Mac‐2 binding protein: Tools to examine protein conformation and function

As part of our effort to identify glycoproteins that contribute to colon cancer progression, we have previously described a family of structurally related glycoproteins expressing β1–6 branched asparagine(Asn)‐linked oligosaccharides defined by monoclonal antibody (MAb 1H9), which are differentially expressed, processed, and glycosylated by human colon carcinoma cell lines (Laferté and Loh [1992]; Biochem J; 283:193–201). MAb 1H9 immunoprecipitates three glycoproteins having apparent sizes of 92–100, 66–70, and 25 kDa, the size heterogeneity attributable to cell‐type specific glycosylation differences. We report on the basis of partial protein and cDNA sequence information, that the 100‐kDa glycoprotein detected by MAb 1H9 is identical to the 90‐kDa glycoprotein variably known as tumor‐associated antigen 90K (TAA90K), Mac‐2 binding protein, and cyclophilin C‐associated protein. Using a PCR‐based cloning strategy, the complete cDNA encoding TAA90K was cloned into the eukaryotic expression vector pCDNA‐3 (pCD‐TAA90K wt ) and the protein expressed in COS‐1 cells. A [ 35 S]methionine‐labeled 60‐kDa polypeptide, processed to an endoglycosidase H‐sensitive 74‐kDa glycoprotein in the presence of dog pancreas microsomes, was detected in a coupled transcription/translation in vitro reaction. The in vitro‐translated 60‐kDa polypeptide and N‐glycanase‐treated TAA90K (60‐kDa species) immunoprecipitated from HT29 cells were shown to be structurally identical by limited proteolytic peptide mapping. Using a new panel of 11 TAA90K‐specific monoclonal antibodies, including five specific for human TAA90K and six cross‐reactive with a 90‐kDa species expressed by COS‐1 cells, we have detected conformational differences between recombinant wild‐type TAA90K, in vitro‐synthesized TAA90K, and mutant forms of TAA90K containing point mutations at residues 189, 223, and 259. Furthermore, we have shown that these mutant forms of TAA90K, as well as a truncated form of TAA90K containing amino acid residues 1–383, are defective in secretion. These studies demonstrate the potential usefulness of TAA90K‐specific monoclonal antibodies for examining the structure and function of TAA90K, and highlight the contribution of specific amino acid residues to its normal processing and secretion. J. Cell. Biochem. 77:540–559, 2000. © 2000 Wiley‐Liss, Inc.