Aczonin, a 550-kD putative scaffolding protein of presynaptic active zones, shares homology regions with Rim and Bassoon and binds profilin.

Neurotransmitter exocytosis is restricted to the active zone, a specialized area of the presynaptic plasma membrane. We report the identification and initial characterization of aczonin, a neuron-specific 550-kD protein concentrated at the presynaptic active zone and associated with a detergent-resistant cytoskeletal subcellular fraction. Analysis of the amino acid sequences of chicken and mouse aczonin indicates an organization into multiple domains, including two pairs of Cys(4) zinc fingers, a polyproline tract, and a PDZ domain and two C2 domains near the COOH terminus. The second C2 domain is subject to differential splicing. Aczonin binds profilin, an actin-binding protein implicated in actin cytoskeletal dynamics. Large parts of aczonin, including the zinc finger, PDZ, and C2 domains, are homologous to Rim or to Bassoon, two other proteins concentrated in presynaptic active zones. We propose that aczonin is a scaffolding protein involved in the organization of the molecular architecture of synaptic active zones and in the orchestration of neurotransmitter vesicle trafficking.

Paralemmin, a prenyl-palmitoyl-anchored phosphoprotein abundant in neurons and implicated in plasma membrane dynamics and cell process formation.

We report the identification and initial characterization of paralemmin, a putative new morphoregulatory protein associated with the plasma membrane. Paralemmin is highly expressed in the brain but also less abundantly in many other tissues and cell types. cDNAs from chicken, human, and mouse predict acidic proteins of 42 kD that display a pattern of sequence cassettes with high inter-species conservation separated by poorly conserved linker sequences. Prenylation and palmitoylation of a COOH-terminal cluster of three cysteine residues confers hydrophobicity and membrane association to paralemmin. Paralemmin is also phosphorylated, and its mRNA is differentially spliced in a tissue-specific and developmentally regulated manner. Differential splicing, lipidation, and phosphorylation contribute to electrophoretic heterogeneity that results in an array of multiple bands on Western blots, most notably in brain. Paralemmin is associated with the cytoplasmic face of the plasma membranes of postsynaptic specializations, axonal and dendritic processes and perikarya, and also appears to be associated with an intracellular vesicle pool. It does not line the neuronal plasmalemma continuously but in clusters and patches. Its molecular and morphological properties are reminiscent of GAP-43, CAP-23, and MARCKS, proteins implicated in plasma membrane dynamics. Overexpression in several cell lines shows that paralemmin concentrates at sites of plasma membrane activity such as filopodia and microspikes, and induces cell expansion and process formation. The lipidation motif is essential for this morphogenic activity. We propose a function for paralemmin in the control of cell shape, e.g., through an involvement in membrane flow or in membrane-cytoskeleton interaction.

Restricted expression of the irreC-rst protein is required for normal axonal projections of columnar visual neurons.

The 104 kDa irreC-rst protein, a member of the immunoglobulin superfamily, mediates homophilic adhesion in cell cultures. In larval optic chiasms, the protein is found on recently formed axon bundles, not on older ones. In developing visual neuropils, it is present in all columnar domains of specific layers. The number of irreC-rst-positive neuropil stratifications increases until the midpupal stage. Immunoreactivity fades thereafter. The functional importance of the restricted expression pattern is demonstrated by the severe projection errors of axons in the first and second optic chiasms in loss of function mutants and in transformants that express the irreC-rst protein globally. Epigenesis of the phenotypes can be explained partially on the bases of homophilic irreC-rst interactions.

The synaptic vesicle-associated protein amphiphysin is the 128-kD autoantigen of Stiff-Man syndrome with breast cancer.

Stiff-Man syndrome (SMS) is a rare disease of the central nervous system (CNS) characterized by progressive rigidity of the body musculature with superimposed painful spasms. An autoimmune origin of the disease has been proposed. In a caseload of more than 100 SMS patients, 60% were found positive for autoantibodies directed against the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD). Few patients, all women affected by breast cancer, were negative for GAD autoantibodies but positive for autoantibodies directed against a 128-kD synaptic protein. We report here that this antigen is amphiphysin. GAD and amphiphysin are nonintrinsic membrane proteins that are concentrated in nerve terminals, where a pool of both proteins is associated with the cytoplasmic surface of synaptic vesicles. GAD and amphiphysin are the only two known targets of CNS autoimmunity with this distribution. This finding suggests a possible link between autoimmunity directed against cytoplasmic proteins associated with synaptic vesicles and SMS.

The irregular chiasm C-roughest locus of Drosophila, which affects axonal projections and programmed cell death, encodes a novel immunoglobulin-like protein.

The axonal projection mutations irregular chiasm C of Drosophila melanogaster comap and genetically interact with the roughest locus, which is required for programmed cell death in the developing retina. We cloned the genomic region in 3C5 by transposon tagging and identified a single transcription unit that produces a major, spatially and temporally regulated mRNA species of approximately 5.0 kb. Postembryonic expression is strong in the developing optic lobe and in the eye imaginal disc. The gene encodes a transmembrane protein of 764 amino acids with five extracellular immunoglobulin-like domains and similarity to the chicken axonal surface glycoprotein DM-GRASP/SC1/BEN. Both known irreC alleles reduce the level of transcription, whereas the roughestCT mutation disrupts the intracellular domain of the protein.

cDNA encoding the chicken ortholog of the mouse dilute gene product. Sequence comparison reveals a myosin I subfamily with conserved C-terminal domains.

We report the cDNA-deduced primary structure of the chicken counterpart of the murine dilute gene product, a member of the myosin I family. Comparison of the chicken and mouse sequences reveals a distinct pattern of domains of high and low sequence conservation. An internal deletion of 25 amino acids probably reflects differential mRNA processing. Compared with other myosin heavy chain molecules, sequence similarity is highest with the MYO2 gene product of Saccharomyces cerevisiae. The MYO2 protein, implicated in vectorial vesicle transport, is homologous to the dilute protein over practically its entire length. In addition, the C-terminal domain of the dilute protein is highly similar to a putative glutamic acid decarboxylase sequence cloned from mouse brain. Alternatively, this closely related clone might represent an isoform of the dilute protein derived from a second gene, potentially involved in genetic conditions related to dilute.

Amphiphysin, a novel protein associated with synaptic vesicles.

To obtain access to novel proteins of the neuronal synapse, we have raised antisera against proteins of synaptic plasma membranes and used them for immunoscreening brain cDNA expression libraries. One of the newly isolated cDNAs encodes an acidic protein of 75 kDa with a distinct architecture of structural domains and multiple potential phosphorylation sites. Light and electron microscopy employing monospecific antisera raised against the expression product indicate a synapse-specific, presynaptic localization of this protein in many synapses of the chicken and rat nervous system. Its overall distribution in brain is very similar to that of synaptophysin, a ubiquitous protein of synaptic vesicles. In addition to brain, the protein or its mRNA is expressed in adrenal gland and anterior and posterior pituitary, but was not detected in a variety of other tissues. In controlled pore glass chromatography the native protein copurifies with synaptic vesicles and largely remains associated with them under various washing conditions. However, its amino acid sequence is very hydrophilic and it segregates into the aqueous phase in detergent phase partition. An earlier step of synaptic vesicle purification, sucrose cushion centrifugation, separates a vesicle-bound fraction of this protein from an unbound fraction. This seems to be a new, perhaps peripheral, protein of synaptic vesicles for which we propose the name, amphiphysin.