Members of the SNARE (soluble N -ethylmaleimide-sensitive fusion protein attachment protein receptor) superfamily [syntaxins, VAMPs (vesicle-associated membrane proteins) and SNAP25 (synaptosome-associated protein-25)-related proteins] are required for intracellular membrane-fusion events in eukaryotes. In neurons, assembly of SNARE core complexes comprising the presynaptic membrane-associated SNAREs syntaxin 1 and SNAP25, and the vesicle-associated SNARE VAMP2, is necessary for synaptic vesicle exocytosis. Several accessory factors have been described that associate with the synaptic SNAREs and modulate core complex assembly or mediate Ca2+ regulation. One such factor, Snapin, has been reported to be a brain-specific protein that interacts with SNAP25, and regulates association of the putative Ca2+-sensor synaptotagmin with the synaptic SNARE complex [Ilardi, Mochida and Sheng (1999) Nat. Neurosci. 2, 119-124]. Here we demonstrate that Snapin is expressed ubiquitously in neuronal and non-neuronal cells. Furthermore, using protein-protein-interaction assays we show that Snapin interacts with SNAP23, the widely expressed homologue of SNAP25, and that the predicted C-terminal helical domain of Snapin contains the SNAP23-binding site. Subcellular localization experiments revealed that Snapin is a soluble protein that exists in both cytosolic and peripheral membrane-bound pools in adipocytes. Moreover, association of Snapin with the plasma membrane was detected in cells overexpressing a Snapin-green fluorescent protein fusion protein. Finally, we show that Snapin is able to form a ternary complex with SNAP23 and syntaxin 4, suggesting that it is a component of non-neuronal SNARE complexes. An important implication of our results is that Snapin is likely to perform a general role in SNARE-mediated vesicle fusion events in non-neuronal cells in addition to its participation in Ca2+-regulated neurosecretion.

All retroviruses contain two strands of RNA that bind together noncovalently to form dimers. The evolutionary conservation of genomic RNA dimerization illustrates its importance in the retroviral life cycle (reviewed in references 19, 51, and 57). The dimeric RNA structure is one of the conformations that human immunodeficiency virus type 1 (HIV-1) genomic RNA assumes, and different structural motifs of retroviral genomic RNA are required at distinct stages of virus production. Hence, RNA structures are thought to be important regulatory elements of HIV-1 replication (12).Dimeric RNA from several different retroviruses has been visualized by electron microscopy and was shown to interact near the 5Ј end of the genome at a region designated the dimer linkage structure (8, 9, 33, 44). In HIV-1, a region important for RNA dimerization termed the dimerization initiation site (DIS) was identified through the use of short, in vitro-transcribed RNA strands (14,34,43,47,65). The DIS is located within the 5Ј untranslated region of the genome and forms a hairpin structure (65). Mutations that prevent formation of the DIS loop self-complementary region prevent the RNA dimerization of in vitro-transcribed RNA strands (47,48,65). Dimerization of genomic RNA is critical for the strand transfer of newly synthesized viral cDNA and template switching during reverse transcription (4, 5, 10), and probably one of the most important functions of genomic RNA dimerization is that it enables the recombination of retroviral genomes during reverse transcription (4,5,71). Regions of the genome involved in RNA dimerization have also been shown to have increased recombination, such as the dimer linkage structure of murine leukemia virus (40) and the DIS in HIV-1 (4, 5). Therefore, the dimeric conformation of RNA may regulate recombination events, which are important in terms of viral fitness, enabling increased genetic variability and rescue of reverse transcription in the event of strand breakage.HIV-1 RNA structural motifs have been shown to have critical regulatory roles at multiple steps of the viral life cycle, including translation control of HIV-1 Gag and Gag-polymerase (Pol) polyproteins via an RNA stem-loop to induce ribosomal frameshifting (28) and transcriptional regulation through the trans activation response element RNA structure (11). In addition, the HIV-1 5Ј untranslated region contains multiple RNA structural motifs that are required for packaging of genomic RNA (37) and reverse transcription of viral genomic RNA (6,7,38). It has been proposed that HIV-1 genomic RNA can also assume alternative conformations that serve as a switch between a translation-competent mRNA and a dimerization and packaging-competent genomic RNA (27). It is thought that the long-distance interaction (LDI) formed by the translation-competent RNA masks the DIS, preventing dimerization. Conversely, the branched multiple hairpin (BMH) structure exposes the DIS promoting dimerization and packaging while occluding the Gag start codon (1). In vitro evidenc...