Polyserase-I, a human polyprotease with the ability to generate independent serine protease domains from a single translation product

Abstract

We have identified and cloned a human liver cDNA encoding an unusual mosaic polyprotein, called polyserase-I (polyserine protease-I). This protein exhibits a complex domain organization including a type II transmembrane motif, a low-density lipoprotein receptor A module, and three tandem serine protease domains. This unusual modular architecture is also present in the sequences predicted for mouse and rat polyserase-I. Human polyserase-I gene maps to 19p13, and its last exon overlaps with that corresponding to the 3 UTR of the gene encoding translocase of mitochondrial inner membrane 13. Northern blot analysis showed the presence of a major polyserase-I transcript of 5.4 kb in human fetal and adult tissues and in tumor cell lines. Analysis of processing mechanisms of polyserase-I revealed that it is synthesized as a membraneassociated polyprotein that is further processed to generate three independent serine protease units. Two of these domains are proteolytically active against synthetic peptides commonly used for assaying serine proteases. These proteolytic activities of the polyserase-I units are blocked by serine protease inhibitors. We show an example of generation of separate serine protease domains from a single translation product in human tissues and illustrate an additional mechanism for expanding the complexity of the human degradome, the entire protease complement of human cells and tissues.

Human Polyserase-2, a Novel Enzyme with Three Tandem Serine Protease Domains in a Single Polypeptide Chain

“…It is remarkable that the fluorescent signal can be detected only if the polyserase-2 transfected cells have been previously permeabilized but is absent if samples were not treated with Triton X-100 (data not shown). This situation clearly differs from that observed in polyserase-1, in which the corresponding fluorescent signal can be detected without permeabilization of the cells, as a consequence of its membrane localization (10). According to this finding, we conclude that polyserase-2 is not a membrane-anchored protein and that it is likely secreted to the extracellular medium.…”

“…Thus, both proteases contain three tandem serine protease domains in their respective amino acid sequences. However, the polyserase-2 architecture is less complex because of the lack of additional domains such as a type II transmembrane sequence and a low-density lipoprotein receptor A module present in polyserase-1 (10). Consistent with these structural differences, experimental analysis demonstrated that polyserase-2 is detected as a soluble protease, whereas polyserase-1 is associated with the plasma membrane through its type II transmembrane sequence.…”

“…Thus, the three protease domains present in polyserase-2 are not cleaved from the original translation product and remain as an integral part of a single polypeptide chain. By contrast, polyserase-1 undergoes a complex series of proteolytic events that lead to the generation of three independent serine protease units (10). Likewise, amphibian oviductin and ovochymase also release their respective protease modules after translation of a single polyprotein product (11)(12)(13).…”

“…Thus, the first domain shows the highest percentage of identity with ␥1-tryptase (42%) (20), pancreasin (42%) (21), different members of the type II transmembrane serine proteinase family such as matriptase (40%) and matriptase-2 (40%) (22)(23)(24), and the three serine protease domains of polyserase-1 (41%) (10). The second domain is most closely related to matriptase-2 (33% identities), prostasin (30%) (25), ⑀-tryptases (29%) (26), and the three serine protease domains of polyserase-1 (29%) (10). Finally, the third domain also shows a similar percentage of identity with these same proteins (30% with prostasin, 29% with matriptase-2, 29% with ⑀-tryptase, 28% with ␥1-tryptase, and 27% with the serine protease domains of polyserase-1).…”

“…Recently, and as part of our studies aimed at characterizing mammalian degradomes (the complete set of proteases present in these organisms), we have identified and cloned a human liver cDNA encoding an unusual mosaic protease called polyserase-1 (polyserine protease-1) (10). This protein exhibits a complex modular architecture composed of a type II transmembrane motif, a low-density lipoprotein receptor A module, and three tandem serine protease domains.…”