|Budget Amount *help
¥3,400,000 (Direct Cost : ¥3,400,000)
Fiscal Year 1999 : ¥1,200,000 (Direct Cost : ¥1,200,000)
Fiscal Year 1998 : ¥2,200,000 (Direct Cost : ¥2,200,000)
One of the 5 MT-MMP members, MT4-MMP, was originally identified by cDNA cloning from breast carcinoma cell line by Puente at al. (Cancer Res. 56, 944-949, 1996). However, the coding region for a signal peptide, which exists in all MMP family members, is substituted with an unknown sequence in the cDNA, and it could not direct expression of the gene product in the transfected cells. In contrast, we isolated a mouse and human mt4-mmp (mt4-mmp(m)) gene that can direct expression of the gene product by having a coding region for a signal peptide. The Puente's sequence was minor transcript representing only 1/10 of the total mt4-mmp mRNA and found to be derived from an aberrant splicing product for the first intron. Thus, we have isolated a major functional transcripts of MT4-MMP.
A putative transmembrane domain of MT4-MMP locates at the very C-terminal end while others have an about 20 aminoacids cytoplasmic domain following the transmembrane domain. Such sequences often act as a glycosyl-phosphatidyl inositol (GPI) anchoring signal rather than as a transmembrane domain, suggesting the possibility that MT4-MMP is a GPI-anchored proteinase. Our result showed that [3H] ethanolamine, that can be incorporated into the GPI unit, specifically labeled the MT4-MMP C-terminal end in a sequence dependent manner. In addition, phosphatidylinositol-specific phospholipase C (PI-PLC) treatment released the MT4-MMP from the surface of transfected cells. These results indicate that MT4-MMP is the first GPI-anchored proteinase in the MMP family. During cultivation of the transfected cells, MT4-MMP appeared to be shed from the cell surface by the action of an endogenous metalloproteinase. GPI-anchoring of MT4-MMP on the cell surface indicates a unique biological function and character for this proteinase.