Magazine
R249
Quick guide
The Sac phosphatase domain William E. Hughes What is it? The Sac domain is a 400
amino acid region that exhibits phosphatidylinositol polyphosphate phosphatase activity.
synaptojanin, the yeast synaptojanins Inp52p and Inp53p as well as Sac1p. Two classes of Sac domain proteins, those like synaptojanin and those more like Sac1p, have now been identified in mammals as well as lower eukaryotes.
It came to prominence… in 1996
when synaptojanin was cloned and shown to participate with dynamin in synaptic vesicle recycling. Synaptojanin is a phosphatidylinositol 5-phosphatase able to remove the phosphate from the 5-position of phosphatidylinositol(4,5)bisphosphat e (PtdIns(4,5)P2). The Sac domain is a region of homology between the amino terminus of synaptojanin and the otherwise unrelated yeast protein Sac1p, which regulates the actin cytoskeleton and phospholipid metabolism. It came to prominence, again… in
1999–2000 when various groups assigned novel phosphatidylinositol polyphosphate phosphatase activity to the Sac domain of human
Is it specific? No… and yes. No — it
can hydrolyse phosphate from any of the three positions of inositol that may be phosphorylated (3-, 4- and 5). However, it is specific in that adjacent phosphates are resistant to hydrolysis. Sac domains cannot hydrolyse phosphate from PtdIns(4,5)P2 or PtdIns(3,4)P2, or PtdIns(3,4,5)P3, but can hydrolyse PtdIns(3,5)P2. How does it work? The Sac domain contains a highly conserved CX5R(T/S) motif, thought to be the catalytic motif of many metalindependent protein and inositide polyphosphate phosphatases. Does it not work sometimes? The Sac
domain from the yeast synaptojanin PtdIns(3,4)P2
Synaptojanins
PtdIns4P
PtdIns
PtdIns5P
PtdIns(4,5)P2
Synaptojanins Sac1p
PtdIns(3,4,5)P3
Kinase Phosphatase Sac phosphatase
PtdIns3P
PtdIns(3,5)P2
5-phosphatase Current Biology
Phosphatidylinositol (PtdIns) is phosphorylated on the inositol ring to form phosphatidylinositol polyphosphates (PtdInsPs). Synaptojanins are able to hydrolyse phosphatidylinositol bisphosphates (PtdIns(4,5)P2) and
PtdIns(3,5)P2) to PtdIns without producing a monophosphorylated intermediate as these proteins have two phosphatase domains. Sac phosphatase domains can also hydrolyse PtdIns(3,5)P2 to produce PtdIns.
Inp51p differs from other Sac domains as the cysteine, arginine and threonine/serine residues within the CX5R(T/S) motif are absent. Interestingly, the Inp51p Sac domain does not exhibit any phosphatase activity. As these are among the few significant changes seen between Inp51p Sac domain and other Sac domains, this is a ‘natural’ confirmation of the role of this motif in catalysis. Specific mutations exist that alter the regulation and substrate specificity of the domain, so we have a pretty good idea how it functions though we haven’t solved the structure of the protein yet. So synaptojanins have two phosphatase domains? Yes, a
5-phosphatase and a Sac phosphatase domain. This is an important feature as each one of the phosphatidylinositol phosphates has important roles in numerous signalling pathways and trafficking processes. The two domains enable synaptojanins to hydrolyse PtdIns(4,5)P2 to PtdIns without producing another signal, PtdIns4P, en route (see Figure). Where can I find out more? Nemoto Y, Kearns BG, Wenk MR, Chen H, Mori K, Alb J, De Camilli P, Bankaitis VA: Functional characterization of a mammalian Sac1 and mutants exhibiting substrate specific defects in phosphoinositide phosphatase activity. J Biol Chem 2000, 275:34293-34305. Hughes WE, Cooke FT, Parker PJ: Sac phosphatase domain proteins. Biochem J 2000, 350: 337-352. Stenmark H: Cycling lipids. Curr Biol 2000, 10:R57-R59 Cockcroft S: Biology of Phosphoinositides. Oxford: Oxford University Press; 2000
Address: Protein Phosphorylation Laboratory, Imperial Cancer Research Fund, 44, Lincoln's Inn Fields, London WC2A 3PX, UK. E-mail:
[email protected]
