Exhibits trypsin-like activity as defined by cleavage of synthetic substrates with Arg or Lys as the P1 site (PubMed:10373424). Involved in the terminal differentiation of keratinocytes through prostasin (PRSS8) activation and filaggrin (FLG) processing (PubMed:18843291). Proteolytically cleaves and therefore activates TMPRSS13 (PubMed:28710277). {ECO:0000269|PubMed:10373424, ECO:0000269|PubMed:18843291, ECO:0000269|PubMed:28710277}. This is the function of ST14 (ST14 transmembrane serine protease matriptase, Ensembl gene identifier ENSG00000149418).