Laboratory of Molecular Biology, National Cancer Institute, Bethesda, MD 20892.
The major excreted protein (MEP) of malignantly transformed mouse fibroblasts is the precursor to an acid proteinase with enzymic specificity similar to that of human cathepsin L. By cross-hybridization with a mouse MEP sequence, cDNA clones of the human form of MEP in an SV40 expression vector were isolated. A 1.6 kb cDNA showed 70% deduced amino acid sequence identity with mouse MEP. The deduced amino acid sequence of the cloned human MEP was the same, except for two amino acids, as the N-terminal sequence of mature human cathepsin L, thereby establishing that human MEP is human pro-(cathepsin L). Use of this human pro-(cathepsin L) cDNA clone allowed the detection of a 1.6-1.8 kb pro-(cathepsin L) mRNA in human cells which was not detected with a mouse pro-(cathepsin L) probe.