LEUKOCYSTATIN, A NEW CLASS-II CYSTATIN EXPRESSED SELECTIVELY BY HEMATOPOIETIC-CELLS

We describe a new cystatin in both mice and humans, which we termed le ukocystatin. This protein has all the features of a Class II secreted inhibitory cystatin but contains lysine residues in the normally hydro phobic binding regions. As determined by cDNA library Southern blots, this cystatin is expressed selectively in hematopoietic cells, althoug h fine details of the distribution among these cell types differ betwe en the human and mouse mRNAs. In addition, we have determined the geno mic organization of mouse leukocystatin, and we found that in contrast to most cystatins, the leukocystatin gene contains three introns. The recombinant proteins corresponding to these cystatins were expressed in Escherichia coli as N-terminal glutathione S-transferase or FLAG(TM ) fusions, and studies showed that they inhibited papain and cathepsin L but with affinities lower than other cystatins. The unique features of leukocystatin suggests that this cystatin plays a role in immune r egulation through inhibition of a unique target in the hematopoietic s ystem.