Proteolytic processing of the type II transmembrane serine protease matriptase-2, the key regulator of iron homeostasis

Matriptase-2 (also known as TMPRSS6) belongs to the type II transmembrane serine proteases (TTSPs) which are localized at the cell surface exhibiting a short cytoplasmic N-terminal tail, a transmembrane domain and a large extracellular C-terminus that contains the non-catalytic stem region and the protease domain. Matriptase-2 was recently identified as a potent negative modulator of the systemic iron-regulatory hormone hepcidin [1,2]. To gain further insight into the molecular regulation of systemic iron homeostasis we characterized the proteolytic processing of human matriptase-2 in detail. By the use of transfected cells we demonstrated that matriptase-2 is synthesized as an inactive membrane-bound single-chain polypeptide that undergoes complex proteolytic processing during zymogen activation [3]. We identified and characterized a released two-chain portion of the protein in the extracellular space that was highly active resulting from three proteolytic processing events: two cleavages occur after Arg⁴⁰⁴ and Arg⁴³⁷, within the stem region, and are important for cell surface release of matriptase-2, and one cleavage occurs after residue Arg⁵⁶⁷, within the conserved activation site, converting the single-chain zymogen into the activated two-chain protease. Additionally, mutation of the catalytic active Ser⁷⁵³ reveals that shedding and activation occur autocatalytically via a trans-mechanism indicating that matriptase-2 acts as an upstream protease.