Syntaxin-1 is a component of the synaptic vesicle docking and/or membrane fusion soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) complex (7S and 20S complexes) in nerve terminals. Syntaxin-1 also forms a heterodimer with Munc18/n-Sec1/rbSec1 in a complex that is distinct from the 7S and 20S complexes. In this report, we identify a novel syntaxin-1-binding protein, tomosyn, that is capable of dissociating Munc18 from syntaxin-1 and forming a novel 10S complex with syntaxin-1, soluble N-etyhlmaleimide-sensitive factor attachment (SNAP) 25, and synaptotagmin. The 130 kDa isoform of tomosyn is specifically expressed in brain, where its distribution partly overlaps with that of syntaxin-1 in nerve terminals. High level expression of either syntaxin-1 or tomosyn results in a specific reduction in Ca2+-dependent exocytosis from PC12 cells. These results suggest that tomosyn is an important component in the neurotransmitter release process where it may stimulate SNARE complex formation.

Rab GTPases and SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) are evolutionarily conserved essential components of the eukaryotic intracellular transport system. Although pairing of cognate SNAREs is sufficient to fuse membranes in vitro, a complete reconstitution of the Rab-SNARE machinery has never been achieved. Here we report the reconstitution of the early endosomal canine Rab5 GTPase, its key regulators and effectors together with SNAREs into proteoliposomes using a set of 17 recombinant human proteins. These vesicles behave like minimal 'synthetic' endosomes, fusing with purified early endosomes or with each other in vitro. Membrane fusion measured by content-mixing and morphological assays requires the cooperativity between Rab5 effectors and cognate SNAREs which, together, form a more efficient 'core machinery' than SNAREs alone. In reconstituting a fusion mechanism dependent on both a Rab GTPase and SNAREs, our work shows that the two machineries act coordinately to increase the specificity and efficiency of the membrane tethering and fusion process.

Abstract-The scavenger receptor class B type I (SR-BI) and its human homologue CLA-1 (CD36 and LIMPII Analogous-1) have recently been identified to bind HDL and mediate the selective uptake of HDL lipids. Tissue distribution of both murine and human receptors is quite similar, in that they are expressed abundantly in liver and steroidogenic tissues. However, expression and function of the human SR-BI (hSR-BI), in the periphery of reverse cholesterol transport such as macrophages, are still unclear. In the present study, we have raised two different kinds of anti-hSR-BI polypeptide antibodies (Abs): one against the extracellular domain and the other against the intracellular domain. We have investigated the expression of hSR-BI mRNA and immunoreactive mass in freshly isolated cultured human monocyte-derived macrophages (hM) and in atherosclerotic lesions. Contrary to the earlier report, hSR-BI mRNA was expressed in cultured hM and markedly upregulated with differentiation, determined by Northern blot and reverse transcriptase-based polymerase chain reaction analyses. The mRNA expression pattern during differentiation of hM was very similar to those of SR class A and another member of SR class B, CD36. Protein expression was confirmed by Western blot analyses with the above Abs to show a major 83-kDa band. Modified lipoproteins such as oxidized LDL and acetylated LDL induced a 5-fold increase in mRNA and protein expression of hSR-BI. Confocal immunofluorescence microscopy demonstrated that hSR-BI immunoreactive mass was detectable as a heterogeneous, punctate staining pattern. Furthermore, immunohistochemical analysis showed that immunoreactive mass of hSR-BI was detected in foam cells in human aortic atherosclerotic lesions and that there was no significant difference of staining patterns between the two Abs. This study clearly demonstrates that hSR-BI is expressed in the lipid-laden macrophages in human atherosclerotic lesions, suggesting that it is very important to know its function and regulation in hM to understand the biological utility of this molecule. (Circ Res. 1999;85:108-116.)

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Rabphilin-3A is a downstream target molecule of Rab3A small GTP-binding protein and implicated in Ca 2؉dependent neurotransmitter release. Here we have isolated a rabphilin-3A-interacting molecule from a human brain cDNA library by the yeast two-hybrid method and identified it to be ␣-actinin, known to cross-link actin filaments into a bundle. ␣-Actinin interacts with the N-terminal region of rabphilin-3A, with which GTPRab3A interacts, and this interaction stimulates the activity of ␣-actinin to cross-link actin filaments into a bundle. The interaction of rabphilin-3A with ␣-actinin is inhibited by guanosine 5-(3-O-thio)triphosphateRab3A. These results suggest that the Rab3A-rabphilin-3A system regulates the ␣-actinin-regulated reorganization of actin filaments. It has been shown that reorganization of actin filaments is also involved in Ca 2؉ -dependent exocytosis. Therefore, rabphilin-3A may serve as a linker for Rab3A and cytoskeleton.

To improve the production of recombinant human antithrombin III (AT-III) in Chinese hamster ovary (CHO) cells, the genes encoding transcription factors, ATF4 (activating transcription factor 4) and XBP-1s (the spliced form of X-box binding protein 1), which were involved in the mammalian unfolded protein response (UPR), were cloned from CHO-K1 cells. Overexpression of ATF4 significantly enhanced the production of recombinant AT-III in CHO 13D-35D cells. The specific rate of AT-III production in the ATF4-overexpressed CHO 13D-35D cells reached approximately 23 pg/cell/day. After 144 h of incubation, the AT-III concentration in the culture supernatant was twofold greater compared to that observed with parental CHO 13D-35D cells. In contrast, ectopic expression of XBP-1s failed to enhance the production of recombinant AT-III in CHO 13D-35D cells. RT-PCR analysis revealed that high levels of XBP-1s mRNA were present in the CHO cells, regardless of ectopic expression of XBP-1s. Our results indicate that overexpression of the UPR transcription factor ATF4 is a promising means for improving the production of secreted protein pharmaceuticals in CHO cells.

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