RESUMO
Phosphoinositide signaling pathways regulate many essential cellular functions including proliferation, differentiation and survival, cytoskeletal organization, and vesicular trafficking. The inositol polyphosphate 5-phosphatases regulate the cellular levels of several bioactive phosphoinositide species. This review describes the structure and function of the 5-phosphatase and Sac1 catalytic domains of these enzymes. The crystal structure of the 5-phosphatase domain has been solved and shares homology with members of the AP endonuclease family. The phosphoinositide polyphosphatase activity of the Sac1 domain, found in some inositol polyphosphate 5-phosphatases, is defined by a motif, CX5 R(T/S), also found in both protein and lipid phosphatases.
Assuntos
Monoéster Fosfórico Hidrolases/química , Monoéster Fosfórico Hidrolases/metabolismo , Sequência de Aminoácidos , Animais , Carbono-Oxigênio Liases/química , Carbono-Oxigênio Liases/genética , Domínio Catalítico , DNA Liase (Sítios Apurínicos ou Apirimidínicos) , Evolução Molecular , Humanos , Inositol Polifosfato 5-Fosfatases , Modelos Moleculares , Dados de Sequência Molecular , Proteínas do Tecido Nervoso/química , Proteínas do Tecido Nervoso/genética , Proteínas do Tecido Nervoso/metabolismo , Monoéster Fosfórico Hidrolases/genética , Estrutura Terciária de Proteína , Transdução de Sinais , Especificidade por SubstratoRESUMO
The budding yeast Saccharomyces cerevisiae has four inositol polyphosphate 5-phosphatase (5-phosphatase) genes, INP51, INP52, INP53, and INP54, all of which hydrolyze phosphatidylinositol (4,5)-bisphosphate. INP54 encodes a protein of 44 kDa which consists of a 5-phosphatase domain and a C-terminal leucine-rich tail, but lacks the N-terminal SacI domain and proline-rich region found in the other three yeast 5-phosphatases. We report that Inp54p belongs to the family of tail-anchored proteins and is localized to the endoplasmic reticulum via a C-terminal hydrophobic tail. The hydrophobic tail comprises the last 13 amino acids of the protein and is sufficient to target green fluorescent protein to the endoplasmic reticulum. Protease protection assays demonstrated that the N terminus of Inp54p is oriented toward the cytoplasm of the cell, with the C terminus of the protein also exposed to the cytosol. Null mutation of INP54 resulted in a 2-fold increase in secretion of a reporter protein, compared with wild-type yeast or cells deleted for any of the SacI domain-containing 5-phosphatases. We propose that Inp54p plays a role in regulating secretion, possibly by modulating the levels of phosphatidylinositol (4,5)-bisphosphate on the cytoplasmic surface of the endoplasmic reticulum membrane.
Assuntos
Retículo Endoplasmático/metabolismo , Monoéster Fosfórico Hidrolases/biossíntese , Sequência de Aminoácidos , Membrana Celular/metabolismo , Clonagem Molecular , Citoplasma/metabolismo , Proteínas de Fluorescência Verde , Inositol Polifosfato 5-Fosfatases , Proteínas Luminescentes/metabolismo , Microscopia Confocal , Modelos Biológicos , Dados de Sequência Molecular , Mutação , Monoéster Fosfórico Hidrolases/química , Monoéster Fosfórico Hidrolases/genética , Regiões Promotoras Genéticas , Estrutura Terciária de Proteína , Proteínas Recombinantes/metabolismo , Saccharomyces cerevisiae/metabolismo , Homologia de Sequência de Aminoácidos , Fatores de Tempo , Água/metabolismoRESUMO
The Saccharomyces cerevisiae inositol polyphosphate 5-phosphatases (Inp51p, Inp52p, and Inp53p) each contain an N-terminal Sac1 domain, followed by a 5-phosphatase domain and a C-terminal proline-rich domain. Disruption of any two of these 5-phosphatases results in abnormal vacuolar and plasma membrane morphology. We have cloned and characterized the Sac1-containing 5-phosphatases Inp52p and Inp53p. Purified recombinant Inp52p lacking the Sac1 domain hydrolyzed phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] and PtdIns(3, 5)P(2). Inp52p and Inp53p were expressed in yeast as N-terminal fusion proteins with green fluorescent protein (GFP). In resting cells recombinant GFP-tagged 5-phosphatases were expressed diffusely throughout the cell but were excluded from the nucleus. Following hyperosmotic stress the GFP-tagged 5-phosphatases rapidly and transiently associated with actin patches, independent of actin, in both the mother and daughter cells of budding yeast as demonstrated by colocalization with rhodamine phalloidin. Both the Sac1 domain and proline-rich domains were able to independently mediate translocation of Inp52p to actin patches, following hyperosmotic stress, while the Inp53p proline-rich domain alone was sufficient for stress-mediated localization. Overexpression of Inp52p or Inp53p, but not catalytically inactive Inp52p, which lacked PtdIns(4,5)P(2) 5-phosphatase activity, resulted in a dramatic reduction in the repolarization time of actin patches following hyperosmotic stress. We propose that the osmotic-stress-induced translocation of Inp52p and Inp53p results in the localized regulation of PtdIns(3,5)P(2) and PtdIns(4,5)P(2) at actin patches and associated plasma membrane invaginations. This may provide a mechanism for regulating actin polymerization and cell growth as an acute adaptive response to hyperosmotic stress.
