A soluble form of K-sam/FGFR2 protein in the culture medium of human gastric cancer cells.

K-SAM/FGFR2 gene encodes a receptor tyrosine kinase which belongs to the fibroblast growth factor receptor family and is amplified and overexpressed in KATO-III gastric cancer cells. To characterize K-sam proteins in cancer cells, anti-K-sam rabbit polyclonal antibody PK1-2 was raised and used for the immunoprecipitation analysis. 135, 125, and 110-kDa transmembrane proteins were detected in KATO-III cell lysate, while a soluble truncated 85-kDa K-sam protein was found in the conditioned medium. The molecular size of the soluble K-sam protein does not match with those predicted from the secreted forms of the K-sam cDNA which have been cloned so far. The soluble K-sam protein was highly N-glycosylated like the transmembrane versions, and N-glycosylation appeared to be necessary for its release.