Neuromuscular junctions contain NP185: the multifunctional protein is located at the presynaptic site

A role for the clathrin assembly domain of AP180 in synaptic vesicle endocytosis

We have used the squid giant synapse to determine whether clathrin assembly by AP180 is important for synaptic vesicle endocytosis. The squid homolog of AP180 encodes a 751 amino acid protein with 40% sequence identity to mouse AP180. Alignment of squid AP180 with other AP180 homologs shows that amino acid identity was highest in the N-terminal inositide-binding domain of the protein and weakest in the C-terminal clathrin assembly domain. Recombinant squid AP180 was able to assemble clathrin in vitro, suggesting a conserved three-dimensional structure that mediates clathrin assembly despite the divergent primary sequence of the C-terminal domain. Microinjection of the C-terminal domains of either mouse or squid AP180 into the giant presynaptic terminal of squid enhanced synaptic transmission. Conversely, a peptide from the C-terminal domain of squid AP180 that inhibited clathrin assembly in vitro completely blocked synaptic transmission when it was injected into the giant presynaptic terminal. This inhibitory effect occurred over a time scale of minutes when the synapse was stimulated at low (0.03 Hz), physiological rates. Electron microscopic analysis revealed several structural changes consistent with the inhibition of synaptic vesicle endocytosis; peptide-injected terminals had far fewer synaptic vesicles, were depleted of coated vesicles, and had a larger plasma membrane perimeter than terminals injected with control solutions. In addition, the remaining synaptic vesicles were significantly larger in diameter. We conclude that the clathrin assembly domain of AP180 is important for synaptic vesicle recycling at physiological rates of activity and that assembly of clathrin by AP180 is necessary for maintaining a pool of releasable synaptic vesicles.

AP180 and AP-2 interact directly in a complex that cooperatively assembles clathrin

Clathrin-coated vesicles are involved in protein and lipid trafficking between intracellular compartments in eukaryotic cells. AP-2 and AP180 are the resident coat proteins of clathrin-coated vesicles in nerve terminals, and interactions between these proteins could be important in vesicle dynamics. AP180 and AP-2 each assemble clathrin efficiently under acidic conditions, but neither protein will assemble clathrin efficiently at physiological pH. We find that there is a direct, clathrin-independent interaction between AP180 and AP-2 and that the AP180-AP-2 complex is more efficient at assembling clathrin under physiological conditions than is either protein alone. AP180 is phosphorylated in vivo, and in crude vesicle extracts its phosphorylation is enhanced by stimulation of casein kinase II, which is known to be present in coated vesicles. We find that recombinant AP180 is a substrate for casein kinase II in vitro and that its phosphorylation weakens both the binding of AP-2 by AP180 and the cooperative clathrin assembly activity of these proteins. We have localized the binding site for AP-2 to amino acids 623-680 of AP180. The AP180/AP-2 interaction can be disrupted by a recombinant AP180 fragment containing the AP-2 binding site, and this fragment also disrupts the cooperative clathrin assembly activity of the AP180-AP-2 complex. These results indicate that AP180 and AP-2 interact directly to form a complex that assembles clathrin more efficiently than either protein alone. Phosphorylation of AP180, by modulating the affinity of AP180 for AP-2, may contribute to the regulation of clathrin assembly in vivo.

Pan1p, yeast eps15, functions as a multivalent adaptor that coordinates protein-protein interactions essential for endocytosis

A genetic screen for factors required for endocytosis in the budding yeast Saccharomyces cerevisiae previously identified PAN1. Pan1p is a homologue of the mammalian protein eps15, which has been implicated in endocytosis by virtue of its association with the plasma membrane clathrin adaptor complex AP-2. Pan1p contains two eps15 homology (EH) domains, a protein-protein interaction motif also present in other proteins that function in membrane trafficking. To address the role of Pan1p and EH domains in endocytosis, a yeast two-hybrid screen was performed using the EH domain-containing region of Pan1p. This screen identified yAP180A, one of two yeast homologues of a class of clathrin assembly proteins (AP180) that exhibit in vitro clathrin cage assembly activity. In vitro binding studies using GST fusion proteins and yeast extracts defined distinct binding sites on yAP180A for Pan1p and clathrin. yAP180 proteins and Pan1p, like actin, localize to peripheral patches along the plasma membrane. Mammalian synaptojanin, a phosphatidylinositol polyphosphate-5-phosphatase, also has been implicated in endocytosis recently, and three synaptojanin-like genes have been identified in yeast. We observed genetic interactions between the yeast SJL1 gene and PAN1, which suggest a role for phosphoinositide metabolites in Pan1p function. Together with other studies, these findings suggest that Pan1p coordinates regulatory interactions between proteins required for both endocytosis and actin-cytoskeleton organization; these proteins include the yAP180 proteins, clathrin, the ubiquitin-protein ligase Rsp5p, End3p, and synaptojanin. We suggest that Pan1p (and by extension eps15) serves as a multivalent adaptor around which dynamic interactions between structural and regulatory components of the endocytic pathway converge.

Developmentally regulated mRNA splicing of clathrin assembly protein 3 (AP-3)

Clathrin assembly protein 3 (AP-3) is a neuron-specific component of clathrin coated vesicles. Because it promotes the assembly of uniform clathrin cages, AP-3 may play a regulatory role in synaptic vesicle recycling. Previously, using the monoclonal antibody MabR-18, we demonstrated that AP-3 expression starts from the embryonic stage and is maintained at high levels from the early postnatal stages through adult. In order to study the expression of AP-3 during early postnatal development at the mRNA level, RT-PCR analysis was performed. We divided the coding region of AP-3 into 10 regions and designed primers to amplify each region. As a result, developmentally regulated splicing sites were found in two regions. In one region, a PCR product with a 108-bp deletion was detected from postnatal day 10 (P10). In the other region, a product with a 15-bp deletion was increased compensating for the decrease of the undeleted product. The expression of isoforms changed mainly from around P7 to P10, whereas the level of AP-3 protein remained relatively constant throughout postnatal development. These results suggest that the expression of AP-3 isoforms with mRNA splicing is developmentally regulated in the brain and may be involved in the maturation of synaptic vesicle recycling.

Inhibition of phospholipase D by clathrin assembly protein 3 (AP3)

In the accompanying paper (Chung, J.-K., Sekiya, F., Kang, H.-S., Lee, C., Han, J.-S., Kim, S. R., Bae, Y. S., Morris, A. J., and Rhee, S. G. (1997) J. Biol. Chem. 272, 15980-15985), synaptojanin is identified as a protein that inhibits phospholipase D (PLD) activity stimulated by ADP-ribosylation factor and phosphatidylinositol 4, 5-bisphosphate (PI(4,5)P2). Here, the purification from rat brain cytosol of another PLD-inhibitory protein that is immunologically distinct from synaptojanin is described, and this protein is identified as clathrin assembly protein 3 (AP3) by peptide sequencing and immunoblot analysis. AP3 binds both inositol hexakisphosphate and preassembled clathrin cages with high affinity. However, neither inositol hexakisphosphate binding nor clathrin cage binding affected the ability of AP3 to inhibit PLD. AP3 also binds to PI(4,5)P2 with low affinity. But the PI(4,5)P2 binding was not responsible for PLD inhibition, because the potency and efficacy of AP3 as an inhibitor of PLD were similar in the absence and presence of PI(4,5)P2. A bacterially expressed fusion protein, glutathione S-transferase-AP3 (GST-AP3), also inhibited PLD with a potency equal to that of brain AP3. The inhibitory effect of AP3 appeared to be the result of direct interaction between AP3 and PLD because PLD bound GST-AP3 in an in vitro binding assay. Using GST fusion proteins containing various AP3 sequences, we found that the sequence extending from residues Pro-290 to Lys-320 of AP3 is critical for both inhibition of and binding to PLD. The fact that AP3 is a synapse-specific protein indicates that the AP3-dependent inhibition of PLD might play a regulatory role that is restricted to the rapid cycling of synaptic vesicles.

Regulation of AP-3 function by inositides. Identification of phosphatidylinositol 3,4,5-trisphosphate as a potent ligand

As part of the growing effort to understand the role inositol phosphates and inositol lipids play in the regulation of vesicle traffic within nerve terminals, we determined whether or not the synapse-specific clathrin assembly protein AP-3 can interact with inositol lipids. We found that soluble dioctanoyl-phosphatidylinositol 3,4,5-trisphosphate (DiC8PtdIns(3,4, 5)P3) was only 7.5-fold weaker a ligand than D-myo-inositol hexakisphosphate in assays that measured the displacement of D-myo-[3H]inositol hexakisphosphate. In functional assays we found that both of these ligands inhibited clathrin assembly, but DiC8-PtdIns(3,4,5)P3 was more potent and exhibited a larger maximal effect. We also examined the structural features of DiC8-PtdIns(3,4, 5)P3 that establish specificity. Dioctanoyl-phosphatidylinositol 3, 4-bisphosphate, which does not have a 5-phosphate, and 4, 5-O-bisphosphoryl-D-myo-inosityl 1-O-(1, 2-O-diundecyl)-sn-3-glycerylphosphate, which does not have a 3-phosphate, were, respectively, 2-fold and 4-fold less potent than DiC8-PtdIns(3,4,5)P3 as inhibitors of clathrin assembly. Deacylation of DiC8-PtdIns(3,4,5)P3 reduced its affinity for AP-3 almost 20-fold, and also dramatically lowered its ability to inhibit clathrin assembly. The deacylated products of the soluble derivatives of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 4, 5-bisphosphate were both not significant inhibitors of clathrin assembly. It therefore appears that the interactions of inositides with AP-3 should not be considered simply in terms of electrostatic effects of the highly charged phosphate groups. Ligand specificity appears also to be mediated by hydrophobic interactions with the fatty-acyl chains of the inositol lipids.

The t(10;11)(p13;q14) in the U937 cell line results in the fusion of the AF10 gene and CALM, encoding a new member of the AP-3 clathrin assembly protein family

The translocation t(10;11)(p13;q14) is a recurring chromosomal abnormality that has been observed in patients with acute lymphoblastic leukemia as well as acute myeloid leukemia. We have recently reported that the monocytic cell line U937 has a t(10;11)(p13;q14) translocation. Using a combination of positional cloning and candidate gene approach, we cloned the breakpoint and were able to show that AF10 is fused to a novel gene that we named CALM (Clathrin Assembly Lymphoid Myeloid leukemia gene) located at 11q14. AF10, a putative transcription factor, had recently been cloned as one of the fusion partners of MLL. CALM has a very high homology in its N-terminal third to the murine ap-3 gene which is one of the clathrin assembly proteins. The N-terminal region of ap-3 has been shown to bind to clathrin and to have a high-affinity binding site for phosphoinositols. The identification of the CALM/AF10 fusion gene in the widely used U937 cell line will contribute to our understanding of the malignant phenotype of this line.

Clathrin binding and assembly activities of expressed domains of the synapse-specific clathrin assembly protein AP-3

We separately expressed the 58-kDa C-terminal, 42-kDa middle, 16-kDa C-terminal, and 33-kDa N-terminal regions of AP-3 (also called F1-20, AP180, NP185, and pp155), and determined their clathrin binding and assembly properties. The 58-kDa C-terminal region of AP-3 is able to bind to clathrin triskelia and assemble them into a homogeneous population of clathrin cages and will also bind to preassembled clathrin cages. The 42-kDa central region of AP-3 can bind to both clathrin triskelia and to clathrin cages, but cannot assemble clathrin triskelia into clathrin cages. The 16-kDa C-terminal region of AP-3 can bind to clathrin cages, but cannot bind to clathrin triskelia or assemble clathrin triskelia into clathrin cages. The clathrin binding activities of the 42-kDa central region and 16-kDa C-terminal region are weaker than the corresponding activity of either the 58-kDa C-terminal region or full-length AP-3. Previous efforts had mapped a clathrin binding site within the N-terminal 33 kDa of AP-3 (Murphy, J. E., Pleasure, I. T., Puszkin, S., Prasad, K., and Keen, J. H. (1991) J. Biol. Chem. 266, 4401-4408; Morris, S. A., Schroder, S., Plessmann, U., Weber, K., and Ungewickell, E. (1993) EMBO J. 12, 667-675). However, although the N-terminal 33 kDa of AP-3 is able to bind to clathrin triskelia (Murphy, J. E., Pleasure, I. T., Puszkin, S., Prasad, K., and Keen, J. H. (1991) J. Biol. Chem. 266, 4401-4408; Ye, W., and Lafer, E. M. (1995) J. Neurosci. Res. 41, 15-26), it does not promote their assembly into clathrin cages (Murphy, J. E., Pleasure, I. T., Puszkin, S., Prasad, K., and Keen, J. H. (1991) J. Biol. CHem. 266, 4401-4408; Ye, W., and Lafer, E. M. (1995) J. Neurosci. Res. 41, 15-26) or bind to preassembled clathrin cages (Ye, W., and Lafer, E. M. (1995) J. Neurosci. Res. 41, 15-26). It appears that the smallest functional unit that carries out all of the reported clathrin binding and assembly properties of AP-3, essentially as well as the full-length protein, is the 58-kDa C-terminal region.