MET-ASE - CLONING AND DISTINCT CHROMOSOMAL LOCATION OF A SERINE-PROTEASE PREFERENTIALLY EXPRESSED IN HUMAN NATURAL-KILLER-CELLS

A cDNA clone encoding a human NK serine protease was obtained by scree ning a lambda-gt10 library from the Lopez NK leukemia with the rat nat ural killer Met-ase (RNK-Met-1) cDNA clone. In Northern blot analysis human Met-ase (Hu-Met-1) cDNA hybridized with a 0.9-kb mRNA in two hum an NK leukemia cell lines, unstimulated human PBMC, and untreated puri fied CD3-CD56+ large granular lymphocytes. Unlike other members of the granzyme family that are highly expressed in activated peripheral T c ells, the Hu-Met-I transcript was barely detected in a population of P MA and ionomycin or IL-2-treated high density T cells. Several in vitr o cultured Burkitt lymphomas, chronic- and promyeloid leukemias, acute lymphoblastic leukemias, and colon and ovarian carcinomas did not exp ress Hu-Met-1 mRNA. Hu-Met-1 mRNA expression in a small number of huma n T cell tumor lines did not correlate with any particular phenotype o r stage of development. The presence of Hu-Met-1 mRNA closely correlat ed with the Met-ase activity of cellular lysates prepared from these v arious human peripheral blood subsets and in vitro cultured cell lines . Met-ase activity detected in whole cell lysates of cytotoxic lymphoc ytes was associated with the cytoplasmic granules of these cells. The nucleotide sequence of the Hu-Met-I cDNA clone encodes a predicted ser ine protease of 257 amino acids. The predicted protein is an active en zyme of 232 amino acids with a calculated unglycosylated m.w. of 27,10 0. Hu-Met-1 is 66% identical to RNK-Met-1 at the amino acid level. The human and rat mature protein sequences conserve the active site His, Asp, and Ser amino acids that form the catalytic triad of serine prote ases, all 8 cysteine residues, and several amino acids critical in the formation of the substrate binding pocket. The gene for the Hu-Met-1 serine protease is located on chromosome 19, which distinguishes it fr om any other member of the human granzyme family.