Intracellular polymerization of the serpin plasminogen activator inhibitor type 2.
Plasminogen activator inhibitor type 2 (PAI-2) is synthesized in two molecular forms: an intracellular, nonglycosylated form and an extracellular, glycosylated form. The bitopological distribution of PAI-2 is caused by an inefficient internal secretion signal. In addition, the secretion efficiency of PAI-2 seems to differ, depending on the cell type, differentiation state, and culture conditions. In recombinant cell clones designed for the synthesis of the secreted form of PAI-2, the fraction of secreted PAI-2 decreased with increasing expression levels. Subcellular fractionation of cell clones with higher expression levels revealed that PAI-2 accumulating in the cell was mainly associated with the organelles of the secretory pathway. Electrophoresis under nondenaturating conditions revealed that the PAI-2 retained at higher expression levels was mainly polymerized. Polymers of PAI-2 were also detected in cytosolic extracts prepared from human placenta and phorbol ester-stimulated U 937 cells, indicating that intracellular polymerization of PAI-2 may occur in the cytosols of cells that normally express PAI-2 under physiological conditions. When purified PAI-2 or cellular extracts were incubated at 37 degrees C for 24 h most of the PAI-2 protein was found to polymerize. Polymer formation was prevented by the addition of synthetic peptides with sequences corresponding to residues P2 to P14 in the reactive center loop of PAI-2 and antithrombin. These synthetic peptides also caused dissociation of prepolymerized purified PAI-2 and PAI-2 polymers in cellular extracts. Incubation with unrelated peptides of the same size had no effect on polymer formation or dissociation of preformed polymers, indicating that polymerization of PAI-2 occurs by the loop-sheet mechanism. Taken together, our data suggest that the wild-type form of PAI-2, like some natural pathological genetic variants of alpha1-antitrypsin, antithrombin, and C1 inhibitor readily polymerizes intracellularly and that polymerization may lead to a reduced secretion efficiency.
Mikus
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Department of Medical Biochemistry and Biophysics, Umeå University, S-901 87 Umeå, Sweden.
Ny
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D000595:Amino Acid Sequence; D000818:Animals; D016466:CHO Cells; D002451:Cell Compartmentation; D006224:Cricetinae; D003600:Cytosol / Q000737:chemistry; D006801:Humans; D046911:Macromolecular Substances; D008969:Molecular Sequence Data; D008970:Molecular Weight; D010455:Peptides / Q000737:chemistry; D010920:Placenta / Q000166:cytology; D017396:Plasminogen Activator Inhibitor 2 / Q000737:chemistry* / Q000378:metabolism; D011108:Polymers; D011485:Protein Binding; D011994:Recombinant Proteins
D046911:Macromolecular Substances; D010455:Peptides; D017396:Plasminogen Activator Inhibitor 2; D011108:Polymers; D011994:Recombinant Proteins