The saposin A-type domain is a ~40 amino acid domain present in the saposin precursor, prosaposin, in the propeptides that are cleaved off in the activation reaction. The domain is named after the small lysosomal proteins, saposins, which serve as sphingolipid hydrolase activator proteins in vertebrates. The mammalian saposins are synthesized as a single precursor molecule (prosaposin) which contains four saposin B-type domains (see <PDOC50015>) yielding the active saposins A, B, C and D after proteolytic cleavage, and two saposin A-type domains in the extremities that are removed in the activation reaction. The saposin A-type domain may play a role in targeting, as propeptides containing the saposin A-type domain of the C-terminus of prosaposin and of the N-terminal part of pulmonary surfactant-associated protein B are involved in the transport to the lysosome and to secretory granules (lamellar bodies, which are lysosomal-like organelles), respectively [1,2].

Some proteins known to contain a saposin A-type domain:

• Mammalian proactivator polypeptide, the saposin precursor (prosaposin) that is processed into saposins A, B, C and D.
• Mammalian pulmonary surfactant-associated protein B (SP-B), a surface tension reducing surfactant secreted by type II epithelial cells.

The profile we developed covers the entire saposin A-type domain.