Serine protease that cleaves after hydrophobic or small residues, provided that Arg or Lys is in position P4: known substrates include SREBF1/SREBP1, SREBF2/SREBP2, BDNF, GNPTAB, ATF6, ATF6B and FAM20C. Cleaves substrates after Arg-Ser-Val-Leu (SREBP2), Arg-His-Leu-Leu (ATF6), Arg-Gly-Leu-Thr (BDNF) and its own propeptide after Arg-Arg-Leu-Leu. Catalyzes the first step in the proteolytic activation of the sterol regulatory element-binding proteins (SREBPs) SREBF1/SREBP1 and SREBF2/SREBP2. Also mediates the first step in the proteolytic activation of the cyclic AMP-dependent transcription factor ATF-6 (ATF6 and ATF6B). Mediates the protein cleavage of GNPTAB into subunit alpha and beta, thereby participating in biogenesis of lysosomes. Cleaves the propeptide from FAM20C which is required for FAM20C secretion from the Golgi apparatus membrane and for enhancement of FAM20C kinase activity, promoting osteoblast differentiation and biomineralization. Involved in the regulation of M6P-dependent Golgi-to-lysosome trafficking of lysosomal enzymes. It is required for the activation of CREB3L2/BBF2H7, a transcriptional activator of MIA3/TANGO and other genes controlling mega vesicle formation. Therefore, it plays a key role in the regulation of mega vesicle-mediated collagen trafficking.