Deep mutational scanning of the plasminogen activator inhibitor-1 functional landscape

The serine protease inhibitor (SERPIN) plasminogen activator inhibitor-1 (PAI-1) is a key regulator of the fibrinolytic system, inhibiting the serine proteases tissue- and urokinase-type plasminogen activator (tPA and uPA, respectively). Missense variants may render PAI-1 non-functional through misfolding, leading to its turnover as a protease substrate, or to a more rapid transition to the latent/inactive state. Deep mutational scanning was performed to evaluate the impact of amino acid sequence variation on PAI-1 inhibition of uPA using an M13 filamentous phage display system. The effects of single amino acid substitutions on PAI-1's functional inhibition of its canonical target proteases, tPA and uPA , have been determined for only a small fraction of potential mutations. To construct a more comprehensive dataset, a mutagenized PAI-1 library, encompassing ~70% of potential single amino acid substitutions, was displayed on M13 filamentous phage. From this library, the relative effects of 27% of all possible missense variants on PAI-1 inhibition of urokinase-type plasminogen activator were determined using high-throughput DNA sequencing with 826 missense variants demonstrating conserved inhibitory activity and 1137 resulting in loss of PAI-1 function. Comparison of these deep mutational scanning results to predictions from PolyPhen-2 and SIFT demonstrate the limitations of these algorithms, consistent with similar reports for other proteins. Comparison to common human PAI-1 gene variants present in the gnomAD database is consistent with evolutionary selection against loss of PAI-1 function. These findings provide insight into structure-function relationships for PAI-1 and other members of the SERPIN superfamily.