Synaptojanin 2, a Novel Synaptojanin Isoform with a Distinct Targeting Domain and Expression Pattern

Abstract

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 aminoterminal 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 M r 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.Inositol metabolism is now considered to play essential roles in membrane trafficking processes within the cell, apart from its well established role in signal transduction (1-3). Of particular importance, distinct phosphoinositides seem to be involved in specific vesicle transport steps, as demonstrated by critical requirement of several enzymes responsible for synthesis and metabolism of phosphoinositides in different membrane trafficking pathways (4).Synaptojanin is a recently identified inositol 5Ј-phosphatase enriched in nerve terminals (5-7). It has a 3-domain structure in which the central inositol 5Ј-phosphatase domain is flanked by a proline-rich domain at the carboxyl-terminal side and by a domain homologous to the yeast protein SacI (8) at the aminoterminal side. Synaptojanin is concentrated in nerve terminals and localized in close proximity to clathrin-and dynamincoated endocytic intermediates (9). Synaptojanin and dynamin 1 are the major interacting proteins for amphiphysin 1, another nerve terminal protein with a putative role in synaptic vesicle endocytosis (7, 10). In addition, synaptojanin, dynamin 1, and amphiphysin 1 undergo parallel dephosphorylation upon nerve terminal depolariz...

Evolutionary conserved longevity genes and human cognitive abilities in elderly cohorts

“…Alternative splicing of SYNJ2 results in multiple transcript variants and is an important candidate considering its role in ubiquitous signal-transduction pathways. 51 Gene Ontology (GO) links five biological processes to SYNJ2 through inferred electronic annotation (brain development (GO: 007420), dephosphorylation (GO: 0016311), inositol phosphate dephosphorylation (GO: 0046855), phosphoinositide dephosphorylation (GO: 0046856) and intracellular distribution of mitochondria (GO: 0048312)). A limitation of our study is the lack of consistent association between cohorts, both at the SNP level, despite using the same SNPs, and at the phenotype level, despite each general factor and intelligence test capturing overlapping cognitive abilities.…”

Phosphoinositides in Constitutive Membrane Traffic

“…Mice deficient in synaptojanin 1 (67,181) and mutants in Unc26, the C. elegans synaptojanin ortholog (134), have neurological defects and nerve endings that accumulate coated vesicles, suggesting that there is a defect in vesicle uncoating. Synaptojanin 2 has a broader tissue distribution and differs from synaptojanin 1 at the COOH terminus (254). Synaptojanin 2 is reported to be an effector for the small GTPase Rac1, and overexpression …”

rSac3, a novel Sac domain phosphoinositide phosphatase, promotes neurite outgrowth in PC12 cells

“…Mice that are deficient in synaptojanin 1 have elevated levels of PI(4,5)P 2 and PI(3,4,5)P 3 in the brain cytosol and accumulate coated vesicles at nerve terminals, suggesting that synaptojanin 1 functions in recycling vesicles back to the synaptic vesicle pool [19]. Synaptojanin 2 is expressed as multiple alternative splice forms in various tissues [12], and may be involved in clathrin-coated pit formation and lipid hydrolysis, which are required during early clathrin-mediated endocytosis [20].…”

“…This family can be further divided into two subfamilies. Proteins in one subfamily contain the Sac domain and the type II phosphatidylinositol phosphatase 5-phosphatase domain, and members include mammalian synaptojanin 1 [11] and synaptojanin 2 [12], and yeast Inp51p, Inp52p and Inp53p [13]. Synaptojanin 1 is enriched in nerve terminals and associates with membranes that are coated with clathrin [11].…”