The SH3p4/Sh3p8/SH3p13 protein family: binding partners for synaptojanin and dynamin via a Grb2-like Src homology 3 domain

Dap160, a neural-specific Eps15 homology and multiple SH3 domain-containing protein that interacts with Drosophila dynamin

The discovery of overlapping hot spots of dynamin (Estes, P. S., Roos, J., van der Bliek, A., Kelly, R. B., Krishnan, K. S., and Ramaswami, M. (1996) J. Neurosci. 16, 5443-5456) and the heterotetrameric adaptor 2 complex (Gonzalez-Gaitan, M., and Jäckle, H. (1997) Cell 88, 767-776) in Drosophila nerve terminals led to the concept of zones of active endocytosis close to sites of active exocytosis. The proline-rich domain of Drosophila dynamin was used to identify and purify a third component of the endocytosis zones. Dap160 (dynamin-associated protein 160 kDa) is a membrane-associated, dynamin-binding protein of 160 kDa that has four putative src homology 3 domains and an Eps15 homology domain, motifs frequently found in proteins associated with endocytosis. The binding capacities of the four putative src homology 3 domains were examined individually and in combination and shown to bind known proteins that contained proline-rich domains. Each binding site, however, was different in its preference for binding partners. We suggest that Dap160 is a scaffolding protein that helps anchor proteins required for endocytosis at sites where they are needed in the Drosophila nerve terminal.

Calcium triggers calcineurin-dependent synaptic vesicle recycling in mammalian nerve terminals

BACKGROUND

Following exocytosis at the synapse, synaptic vesicle components are recovered by endocytosis. Morphological analysis has suggested that this occurs by a clathrin-mediated pathway, and the GTPase dynamin is thought to be involved in 'pinching off' endocytosing vesicles. The finding that the calcium-dependent phosphatase calcineurin can dephosphorylate dynamin and two other proteins implicated in endocytosis (amphiphysin and synaptojanin) has suggested a potential role for calcium and dephosphorylation in regulating synaptic vesicle endocytosis.

RESULTS

We tested this hypothesis with an endocytosis assay in isolated nerve terminals (synaptosomes) that relies on the use of the fluorescent dye FM2-10. In synaptosomes, vesicle recycling occurs predominantly via a pathway dependent on both dynamin and amphiphysin. We found that endocytosis could be stimulated maximally at calcium concentrations that yielded only low levels of exocytosis, suggesting that the two processes had different calcium sensitivities cyclosporin A and Fk506, we identified calcineurin as a calcium sensor for endocytosis and showed that its activity is essential for synaptic vesicle endocytosis in synaptosomes.

CONCLUSIONS

Our results suggest that dynamin-dependent synaptic vesicle endocytosis is triggered by calcium influx occurring upon nerve-terminal depolarisation. An essential mediator of calcium's effect is calcineurin, the activation of which leads to dephosphorylation of at least four proteins implicated in endocytosis-dynamin, amphiphysin 1, amphiphysin 2 and synaptojanin. Our findings also imply that endocytosis and exocytosis may occur in tandem in vivo simply because they share a responsiveness to calcium influx, rather than because they are mechanistically coupled.

Molecular cloning of multiple isoforms of synaptojanin 2 and assignment of the gene to mouse chromosome 17A2-3.1

Synaptojanin 2 is an inositol polyphosphate 5'-phosphatase that appears to be regulated by alternative splicing. By screening mouse cDNA libraries derived from either mouse day 16 embryo or adult liver, we have identified additional synaptojanin 2 cDNAs that represent six new isoforms of the protein. This finding, together with other reports, indicates the presence of eight isoforms of synaptojanin 2. Sequence analysis of our cDNA clones suggests that there are at least two putative initiation sites and at least six different sequences coding for the carboxyl-terminus of the molecule. In addition, we have mapped synaptojanin 2 to mouse chromosome 17 band A2-3.1 by fluorescence in situ hybridization.

Probable mechanisms underlying interallelic complementation and temperature-sensitivity of mutations at the shibire locus of Drosophila melanogaster

The shibire locus of Drosophila melanogaster encodes dynamin, a GTPase required for the fission of endocytic vesicles from plasma membrane. Biochemical studies indicate that mammalian dynamin is part of a complex containing multiple dynamin subunits and other polypeptides. To gain insight into sequences of dynamin critical for its function, we have characterized in detail a collection of conditional and lethal shi alleles. We describe a probable null allele of shi and show that its properties are distinct from those of two classes of lethal alleles (termed I and II) that show intergroup, interallelic complementation. Sequenced class I alleles, which display dominant properties, carry missense mutations in conserved residues in the GTPase domain of dynamin. In contrast, the sequenced class II alleles, which appear completely recessive, carry missense mutations in conserved residues of a previously uncharacterized "middle domain" that lies adjacent to the GTPase region. These data suggest that critical interactions mediated by this middle domain are severely affected by the class II lethal mutations; thus, the mutant sequences should be very useful for confirming the in vivo relevance of interactions observed in vitro. Viable heteroallelic combinations of shi lethals show rapid and reversible temperature-sensitive paralytic phenotypes hitherto only described for the ts alleles of shi. When taken together with the molecular analysis of shi mutations, these observations suggest that the GTPase domain of dynamin carries an intrinsically temperature-sensitive activity: hypomorphic mutations that reduce this activity at low temperatures result in conditional temperature-sensitive phenotype. These observations explain why screens for conditional paralytic mutants in Drosophila inevitably recover ts alleles of shi at high frequencies.

Synaptojanin 1: localization on coated endocytic intermediates in nerve terminals and interaction of its 170 kDa isoform with Eps15

Synaptojanin 1 is an inositol 5-phosphatase with a putative role in clathrin-mediated endocytosis. Goal of this study was to provide new evidence for this hypothesis. We show that synaptojanin 1 is concentrated at clathrin-coated endocytic intermediates in nerve terminals. Furthermore, we report that synaptojanin-170, an alternatively spliced isoform of synaptojanin 1, binds Eps15, a clathrin coat-associated protein. Binding is mediated by the COOH-terminal region of synaptojanin-170 which we show here to be poorly conserved from rat to humans, but to contain in both species three asparagine-proline-phenylalanine (NPF) repeats. This motif has been found to be the core of the binding site for the EH domains of Eps15. Together with previous data, our results suggest that synaptojanin 1 can be recruited to clathrin-coated pits via a multiplicity of interactions.

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.

Synaptic vesicle endocytosis

Exocytosis of synaptic vesicles is followed rapidly by reinternalization and recycling of their membranes. Recent studies have confirmed the key role of clathrin-mediated endocytosis in synaptic vesicle reformation and have identified new proteins that participate in this process. In addition, growing evidence suggests that lipids, primarily phosphoinositides, play an important role in synaptic vesicle recycling.