Synaptojanin 2, a novel synaptojanin isoform with a distinct targeting domain and expression pattern.
Synaptojanin (synaptojanin 1) is a recently identified inositol 5'-phosphatase, which is highly enriched in nerve terminals and is implicated in synaptic vesicle recycling. It is composed of three domains: an amino-terminal SacI homology region, a central inositol 5'-phosphatase homology region, and a carboxyl-terminal proline-rich region. We have now identified and characterized a novel form of synaptojanin, synaptojanin 2, which has a broader tissue distribution. Synaptojanin 2 cDNA from rat brain library encodes a protein of 1,248 amino acids with a predicted Mr of 138,268. The two synaptojanin isoforms share 57.2 and 53.8% amino acid identity in their SacI and phosphatase domains, respectively. In marked contrast, their carboxyl-terminal proline-rich regions bear little homology. Expression of synaptojanin 2 in COS7 cells produced a 140-kDa protein with inositol 5'-phosphatase actvity. Protein binding assays demonstrated that among the major src homology 3-proteins known to bind to the proline-rich region of synaptojanin 1, Grb2, amphiphysin, and members of SH3p4/8/13 protein family, only Grb2 bound to that of synaptojanin 2. Furthermore, subcellular fractionation studies in transfected Chinese hamster ovary cells revealed that synaptojanin 2 was predominantly associated with the particulate fraction while synaptojanin 1 was mainly localized in the soluble fraction. This observation suggests that the proline-rich regions of synaptojanins 1 and 2 are implicated in different protein-protein interactions and direct the two isoforms to different subcellular compartments. Our results demonstrate the presence of a family of synaptojanin-type inositol 5'-phosphatases with different tissue and subcellular distributions, which may be involved in distinct membrane trafficking and signal transduction pathways in mammalian cells.
Nemoto
Y
Y
Department of Cell Biology and Howard Hughes Medical Institute, Yale University, School of Medicine, New Haven, Connecticut 06510, USA.
Arribas
M
M
Haffner
C
C
DeCamilli
P
P
D000595:Amino Acid Sequence; D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D018076:DNA, Complementary / Q000737:chemistry; D004791:Enzyme Inhibitors / Q000737:chemistry* / Q000378:metabolism; D018604:Epitope Mapping; D007527:Isoenzymes / Q000737:chemistry* / Q000378:metabolism; D008969:Molecular Sequence Data; D008970:Molecular Weight; D009419:Nerve Tissue Proteins / Q000737:chemistry* / Q000378:metabolism; D000072183:Phosphatidylinositol-3,4,5-Trisphosphate 5-Phosphatases; D010739:Phospholipase D / Q000037:antagonists & inhibitors*; D010744:Phosphoric Monoester Hydrolases / Q000737:chemistry* / Q000378:metabolism; D011485:Protein Binding; D051381:Rats; D016415:Sequence Alignment; D014018:Tissue Distribution; D018909:src Homology Domains
D018076:DNA, Complementary; D004791:Enzyme Inhibitors; D007527:Isoenzymes; D009419:Nerve Tissue Proteins; C097702:synaptojanin; D010744:Phosphoric Monoester Hydrolases; C098706:INPPL1 protein, human; D000072183:Phosphatidylinositol-3,4,5-Trisphosphate 5-Phosphatases; D010739:Phospholipase D