A developmentally regulated mRNA from 3T3 adipocytes encodes a novel serine protease homologue

We previously have isolated cDNA clones for several mRNAs that increase in abundance during the differentiation of 3T3 adipocytes but whose physiological role is unknown. We show here that a mRNA that is complementary to one of these clones and encodes a protein of 28 kDa is expressed abundantly in mouse fat pads but not in several other mouse tissues. Sequence analysis of the corresponding cDNA clone indicated that the encoded protein shows 30% overall amino acid homology to several serine proteases including trypsin, chymotrypsin, and elastase. Homology is much higher (64%) between the 28-kDa protein and regions that are strongly conserved among the members of the serine protease family. The derived protein also has key features characteristic of active serine proteases, including the histidine, aspartic acid, and serine residues, which comprise the charge relay system, and a potential cleavage site for activation of the zymogen. Primer extension analysis performed to obtain the sequence of the 5' end of mRNA that encodes the 28-kDa protein indicates that two forms of this mRNA exist and probably arise through alternative splicing. The two mRNAs encode signal sequences that differ by the deletion of one amino acid near the predicted cleavage site of the signal peptide. These results demonstrate that adipocyte differentiation is accompanied by the expression of mRNA encoding a serine protease homologue that can be synthesized with two different signal peptides.