{PDOC51110}
{PS51110; SAP_A}
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* Saposin A-type domain profile *
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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.

-Sequences known to belong to this class detected by the profile: ALL.
-Other sequence(s) detected in Swiss-Prot: NONE.

-Last update: April 2005 / First entry.

[ 1] Lefrancois S., May T., Knight C., Bourbeau D., Morales C.R.
     "The lysosomal transport of prosaposin requires the conditional
     interaction of its highly conserved d domain with sphingomyelin."
     J. Biol. Chem. 277:17188-17199(2002).
     PubMed=11856752; DOI=10.1074/jbc.M200343200
[ 2] Lin S., Akinbi H.T., Breslin J.S., Weaver T.E.
     "Structural requirements for targeting of surfactant protein B (SP-B)
     to secretory granules in vitro and in vivo."
     J. Biol. Chem. 271:19689-19695(1996).
     PubMed=8702672

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