title: SAC1-like domains of yeast SAC1, INP52, and INP53 and of human synaptojanin encode polyphosphoinositide phosphatases.
abstract: The SAC1 gene product has been implicated in the regulation of actin cytoskeleton, secretion from the Golgi, and microsomal ATP transport; yet its function is unknown. Within SAC1 is an evolutionarily conserved 300-amino acid region, designated a SAC1-like domain, that is also present at the amino termini of the inositol polyphosphate 5-phosphatases, mammalian synaptojanin, and certain yeast INP5 gene products. Here we report that SAC1-like domains have intrinsic enzymatic activity that defines a new class of polyphosphoinositide phosphatase (PPIPase). Purified recombinant SAC1-like domains convert yeast lipids phosphatidylinositol (PI) 3-phosphate, PI 4-phosphate, and PI 3,5-bisphosphate to PI, whereas PI 4,5-bisphosphate is not a substrate. Yeast lacking Sac1p exhibit 10-, 2.5-, and 2-fold increases in the cellular levels of PI 4-phosphate, PI 3,5-bisphosphate, and PI 3-phosphate, respectively. The 5-phosphatase domains of synaptojanin, Inp52p, and Inp53p are also catalytic, thus representing the first examples of an inositol signaling protein with two distinct lipid phosphatase active sites within a single polypeptide chain. Together, our data provide a long sought mechanism as to how defects in Sac1p overcome certain actin mutants and bypass the requirement for yeast phosphatidylinositol/phosphatidylcholine transfer protein, Sec14p. We demonstrate that PPIPase activity is a key regulator of membrane trafficking and actin cytoskeleton organization and suggest signaling roles for phosphoinositides other than PI 4,5-bisphosphate in these processes. Additionally, the tethering of PPIPase and 5-phosphatase activities indicate a novel mechanism by which concerted phosphoinositide hydrolysis participates in membrane trafficking.
authors.affiliation: Departments of Pharmacology and Cancer Biology and of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710, USA.
authors.forename: S
authors.initials: S
authors.lastname: Guo
authors.forename: L E
authors.initials: LE
authors.lastname: Stolz
authors.forename: S M
authors.initials: SM
authors.lastname: Lemrow
authors.forename: J D
authors.initials: JD
authors.lastname: York
mesh_terms: D005656:Fungal Proteins / Q000235:genetics* / Q000378:metabolism; D006801:Humans; D000072181:Inositol Polyphosphate 5-Phosphatases; D009419:Nerve Tissue Proteins / Q000235:genetics* / Q000378:metabolism; D010744:Phosphoric Monoester Hydrolases / Q000235:genetics* / Q000378:metabolism; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000201:enzymology / Q000235:genetics*
chemical_list: D005656:Fungal Proteins; D009419:Nerve Tissue Proteins; D011994:Recombinant Proteins; C097702:synaptojanin; D010744:Phosphoric Monoester Hydrolases; D000072181:Inositol Polyphosphate 5-Phosphatases