Serine peptidase whose precise substrate specificity remains unclear (PubMed:16143824, PubMed:16385448, PubMed:28726805). Does not cleave peptides after a arginine or lysine residue (PubMed:16143824). Regulates trans-Golgi network morphology and sorting by regulating the membrane binding of the AP-1 complex (PubMed:23321636). May play a role in the regulation of synaptic vesicle exocytosis (PubMed:24610330). Inhibited by PMSF and Prefabloc, as well as leupeptin at high concentrations (PubMed:16385448). Partially inhibited by TPCK, a chymotrypsin inhibitor and E64, a cysteine protease inhibitor (PubMed:16385448). Not affected by 4-amidinophenyl-methanesulfonyl fluoride (APMSF), pepstatin or EDTA (PubMed:16385448). Inhibited by 1-isobutyl-3-oxo-3,5,6,7-tetrahydro-2H-cyclopenta[c]pyridine-4-carbonitrile (PubMed:28726805). Homodimer (PubMed:16143824). Interacts with the AP-1 complex (PubMed:23321636). Co-localizes with AP-1 in the trans-Golgi network (By similarity). Co-localizes with MAP2 and ACTB on the cytoskeleton (By similarity). Co-localizes with STX6 and GOSR2 at the Golgi apparatus (By similarity). Expressed in pyramidal neurons of the temporal cortex and neocortex (at protein level) (PubMed:23485813). Widely expressed (PubMed:15913950, PubMed:16385448). Expressed at higher level in brain, skeletal muscle, heart and kidney (PubMed:15913950, PubMed:16385448). Expressed at the endplates in the neuromuscular junction (PubMed:24610330). The gene represented in this entry is involved in disease pathogenesis. Hypotonia-cystinuria syndrome is a contiguous gene syndrome caused by a homozygous deletion on chromosome 2p21 that disrupts the gene represented in this entry and SLC3A1 (PubMed:16385448, PubMed:21686663). A homozygous 77.4-kb deletion that disrupts the gene represented in this entry, SLC3A1 and CAMKMT, causes atypical hypotonia-cystinuria syndrome, characterized by mild to moderate intellectual disability and respiratory chain complex IV deficiency (PubMed:21686663). Patient cells exhibit a larger trans-Golgi network and a reduced redistribution of AP-1 complex, which causes impairment in AP-1 mediated membrane-cytoplasm recycling and secretion (PubMed:23321636). The disease is caused by variants affecting the gene represented in this entry. Belongs to the peptidase S9A family. Extended N-terminus.