Identification of an alternatively spliced seprase mRNA that encodes a novel intracellular isoform

Seprase is a homodimeric 170-kDa integral membrane gelatinase that is relat ed to the ectoenzyme dipeptidyl peptidase IV. We have identified an alterna tively spliced seprase messenger from the human melanoma cell line LOX that encodes a novel truncated isoform, seprase-s. The splice variant mRNA is g enerated by an out-of-frame deletion of a 1223-base pair exonic region that encodes part of the cytoplasmic tail, transmembrane, and the membrane prog imal-central regions of the extracellular domain (Val(5) through Ser(412)) of the seprase 97-kDa subunit (seprase-l). The seprase-s mRNA has an elonga ted 5' leader (548 nucleotides) that harbors at least two upstream open rea ding frames that inhibit seprase-s expression from a downstream major open reading frame. Deletion mutagenesis of the wild type splice variant cDNA co nfirms that initiation of the seprase-s coding sequence begins with an ATG codon that corresponds to Met(522) Of seprase-l, The seprase-s open reading frame encodes a 239-amino acid polypeptide with an M-r similar to 27,000 t hat precisely overlaps the carboxyl-terminal catalytic region of seprase-l.