Altered trafficking of lysosomal proteins in Hermansky-Pudlak syndrome due to mutations in the beta 3A subunit of the AP-3 adaptor.
Hermansky-Pudlak syndrome (HPS) is a genetic disorder characterized by defective lysosome-related organelles. Here, we report the identification of two HPS patients with mutations in the beta 3A subunit of the heterotetrameric AP-3 complex. The patients' fibroblasts exhibit drastically reduced levels of AP-3 due to enhanced degradation of mutant beta 3A. The AP-3 deficiency results in increased surface expression of the lysosomal membrane proteins CD63, lamp-1, and lamp-2, but not of nonlysosomal proteins. These differential effects are consistent with the preferential interaction of the AP-3 mu 3A subunit with tyrosine-based signals involved in lysosomal targeting. Our results suggest that AP-3 functions in protein sorting to lysosomes and provide an example of a human disease in which altered trafficking of integral membrane proteins is due to mutations in a component of the sorting machinery. (+info)
AP-4, a novel protein complex related to clathrin adaptors.
Here we report the identification and characterization of AP-4, a novel protein complex related to the heterotetrameric AP-1, AP-2, and AP-3 adaptors that mediate protein sorting in the endocytic and late secretory pathways. The key to the identification of this complex was the cloning and sequencing of two widely expressed, mammalian cDNAs encoding new homologs of the adaptor beta and sigma subunits named beta4 and sigma4, respectively. An antibody to beta4 recognized in human cells an approximately 83-kDa polypeptide that exists in both soluble and membrane-associated forms. Gel filtration, sedimentation velocity, and immunoprecipitation experiments revealed that beta4 is a component of a multisubunit complex (AP-4) that also contains the sigma4 polypeptide and two additional adaptor subunit homologs named mu4 (mu-ARP2) and epsilon. Immunofluorescence analyses showed that AP-4 is associated with the trans-Golgi network or an adjacent structure and that this association is sensitive to the drug brefeldin A. We propose that, like the related AP-1, AP-2, and AP-3 complexes, AP-4 plays a role in signal-mediated trafficking of integral membrane proteins in mammalian cells. (+info)
Characterization of a single-copy Arabidopsis gene encoding a protein showing limited similarity to the N-terminus of the mammalian clathrin-assembly protein AP180.
The Arabidopsis 194 gene encoding a protein containing sequence similarity to an N-terminal region of the clathrin-assembly protein AP180 has been identified in a 4.9-kb region of genomic DNA upstream of the gene encoding the high mobility group protein HMG-I/Y. The gene consists of 12 exons and 11 introns, identified by comparison with partial cDNAs and using the NetPlantGene programme, and encodes a protein of 584 amino acid residues. The C-terminal region of the protein contains 8 tandem repeats of a 17-amino-acid-residue sequence. Southern blot analysis of genomic DNA from Columbia and Landsberg ecotypes of Arabidopsis indicates the presence of a single copy of the 194 gene. The 194 gene is expressed in all organs of Arabidopsis including roots, stems, leaves, flowers, and developing siliques, as determined by northern blot analysis. (+info)
Endocytosis: an assembly protein for clathrin cages.
The protein AP180 is known to have clathrin-assembly activity in vitro. AP180 has now been found to be crucial for synaptic vesicle endocytosis and the maintenance of a uniform-size vesicle population in vivo. These results significantly advance our understanding of clathrin-mediated endocytosis in the synapse and elsewhere. (+info)
Abnormal expression and subcellular distribution of subunit proteins of the AP-3 adaptor complex lead to platelet storage pool deficiency in the pearl mouse.
The pearl mouse is a model for Hermansky Pudlak Syndrome (HPS), whose symptoms include hypopigmentation, lysosomal abnormalities, and prolonged bleeding due to platelet storage pool deficiency (SPD). The gene for pearl has recently been identified as the beta3A subunit of the AP-3 adaptor complex. The objective of these experiments was to determine if the expression and subcellular distribution of the AP-3 complex were altered in pearl platelets and other tissues. The beta3A subunit was undetectable in all pearl cells and tissues. Also, expression of other subunit proteins of the AP-3 complex was decreased. The subcellular distribution of the remaining AP-3 subunits in platelets, macrophages, and a melanocyte-derived cell line of pearl mice was changed from the normal punctate, probably endosomal, pattern to a diffuse cytoplasmic pattern. Ultrastructural abnormalities in mutant lysosomes were likewise apparent in mutant kidney and a cultured mutant cell line. Genetically distinct mouse HPS models had normal expression of AP-3 subunits. These and related experiments strongly suggest that the AP-3 complex regulates the biogenesis/function of organelles of platelets and other cells and that abrogation of expression of the AP-3 complex leads to platelet SPD. (+info)
UNC-11, a Caenorhabditis elegans AP180 homologue, regulates the size and protein composition of synaptic vesicles.
The unc-11 gene of Caenorhabditis elegans encodes multiple isoforms of a protein homologous to the mammalian brain-specific clathrin-adaptor protein AP180. The UNC-11 protein is expressed at high levels in the nervous system and at lower levels in other tissues. In neurons, UNC-11 is enriched at presynaptic terminals but is also present in cell bodies. unc-11 mutants are defective in two aspects of synaptic vesicle biogenesis. First, the SNARE protein synaptobrevin is mislocalized, no longer being exclusively localized to synaptic vesicles. The reduction of synaptobrevin at synaptic vesicles is the probable cause of the reduced neurotransmitter release observed in these mutants. Second, unc-11 mutants accumulate large vesicles at synapses. We propose that the UNC-11 protein mediates two functions during synaptic vesicle biogenesis: it recruits synaptobrevin to synaptic vesicle membranes and it regulates the size of the budded vesicle during clathrin coat assembly. (+info)
Clathrin functions in the absence of heterotetrameric adaptors and AP180-related proteins in yeast.
The major coat proteins of clathrin-coated vesicles are the clathrin triskelion and heterotetrameric associated protein (AP) complexes. The APs are thought to be involved in cargo capture and recruitment of clathrin to the membrane during endocytosis and sorting in the trans-Golgi network/endosomal system. AP180 is an abundant coat protein in brain clathrin-coated vesicles, and it has potent clathrin assembly activity. In Saccharomyces cerevisiae, there are 13 genes encoding homologs of heterotetrameric AP subunits and two genes encoding AP180-related proteins. To test the model that clathrin function is dependent on the heterotetrameric APs and/or AP180 homologs, yeast strains containing multiple disruptions in AP subunit genes, as well as in the two YAP180 genes, were constructed. Surprisingly, the AP deletion strains did not display the phenotypes associated with clathrin deficiency, including slowed growth and endocytosis, defective late Golgi protein retention and impaired cytosol to vacuole/autophagy function. Clathrin-coated vesicles isolated from multiple AP deletion mutants were morphologically indistinguishable from those from wild-type cells. These results indicate that clathrin function and recruitment onto membranes are not dependent upon heterotetrameric adaptors or AP180 homologs in yeast. Therefore, alternative mechanisms for clathrin assembly and coated vesicle formation, as well as the role of AP complexes and AP180-related proteins in these processes, must be considered. (+info)
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. (+info)