Fusion bundle

PVC Fusion (Gelation). The PVC piimaiy particle flow units (biUion molecule bundles) can partially melt, freeing some molecules of PVC... 

SNARE motifs spontaneously assemble into SNARE complexes. These consist of a bundle of four intertwined a-helices that are connected by a total of 16 layers of mostly hydrophobic amino acid side chains. In the middle of the bundle, there is a highly conserved and polar 0-layer consisting of three glutamine and one arginine residue. These residues are among the most conserved in the SNARE superfamily and led to a classification of SNAREs into Q- and R-SNAREs, respectively. Different fusion steps require different sets of SNAREs but some SNAREs can participate in different complexes, and some fusion steps involve several SNARE complexes that appear to operate in parallel and independently. 

Melikyan GB, Markosyan RM, Hemmatl H, Delmedico MK, Lambert DM, Cohen FS. Evidence that the transition of HIV-1 gp41 into a six-helix bundle, not the bundle configuration, induces membrane fusion. J Cell Biol 2000 151(2) 413 124. 

Markosyan RM, Cohen FS, Melikyan GB. HIV-1 envelope proteins complete their folding into six-helix bundles immediately after fusion pore formation. Mol Biol Cell 2003 14(3) 926—938. 

Myosin-6 is an unusual isoform as it moves in the opposite direction along actin filaments, as compared to all other well-characterized myosins. Myosin-6 is located near the base of the hair bundle, and could play a role in anchoring the apical membrane to prevent fusion along... 

Microfilaments of F actin traverse the microvilli in ordered bundles. The microfila-ments are attached to each other by actin-as-sociated proteins, particularly fimbrin and vil-lin. Calmodulin and a myosin-like ATPase connect the microfilaments laterally to the plasma membrane. Fodrin, another microfila-ment-associated protein, anchors the actin fibers to each other at the base, as well as attaching them to the cytoplasmic membrane and to a network of intermediate filaments. In this example, the microfilaments have a mainly static function. In other cases, actin is also involved in dynamic processes. These include muscle contraction (see p. 332), cell movement, phagocytosis by immune cells, the formation of microspikes and lamellipo-dia (cellular extensions), and the acrosomal process during the fusion of sperm with the egg cell. 

Skeletal muscles consist of bundles of long muscle fibers, which are single cells of diameter 10-100 pm formed by the fusion of many embryonic cells. The lengths are typically 2-3 cm in mammals but may sometimes be as great as 50 cm. Each fiber contains up to 100-200 nuclei. Typical cell organelles are present but are often given special names. Thus, the plasma membrane (plasmalemma) of muscle fibers is called the sarcolemma. The cytoplasm is sarcoplasm, and mitochondria may be called sarcosomes. The major characteristic of muscle is the presence of the contractile myofibrils, organized bundles of proteins 1-2 pm in diameter and not separated by membranes from the cytoplasm. Since they occupy most of the cytoplasm, a substantial number of myofibrils are present in each muscle fiber. 

Fig. 1 Structure of the neuronal SNAREs. Upper panel domain structure of the three neuronal SNARE proteins involved in synaptic vesicle fusion. Syntaxin 1A and SNAP-25 (contains two SNARE motifs) are associated with the presynaptic membrane, whereas synaptobrevin 2 is synaptic vesicle associated. The SNARE motifs form a stable complex (core complex) whose crystal structure has been analyzed (lower panel). In the complex, each of the SNARE motifs adopts an alpha-helical structure, and the four alpha-helices are aligned in parallel forming a twisted bundle (modified from Sutton et al. 1998). Stability of the complex is mediated by layers of interaction (—7 to +8) in which amino acids from each of the four alpha-helices participate (see text).

The major breakthrough in our structural knowledge of the HIV-1 envelope glycoprotein came in 1997 when the X-ray structure of the fusion-active form of gp41 was reported (35-37). Because it is inherently difficult to crystallize envelope glycoproteins, a protein-dissection approach was applied to derive substructures from the N- and C-terminal regions of the ectodomain termed N-36 and C-34. The structure indicates that the N-36/C-34 complex forms a hexahelical bundle consisting of a parallel trimeric coil-coil... 

Jiang S, Lu H, Liu S et al (2004) N-substituted pyrrole derivatives as novel human immunodeficiency virus type 1 entry inhibitors that interfere with the gp41 six-helix bundle formation and block virus fusion. Antimicrob Agents Chemother 48 4349-4359... 

Figure 5.12. Lateral fusion of collagen fibrils during fascicle development of chick extensor tendon. Transmission electron micrograph showing the lateral fusion of collagen fibrils at day 17 of chick embryogenesis. Note that the demarcation between collagen fibrils (arrows) is less clear compared to the cross section shown at day 14 (Figure 5.11). Several fibrils appear to be in the process of fusion generating fibrils with irregular cross sections. The fibril bundle (fiber) diameter is still about 2 pm before fusion similar to that observed on day 14 (see Silver et al., 2003).

A programme is underway to calculate multi-configuration intermediate coupling dielectronic recombination rate coefficients from the (ground plus) metastable levels of an ion to all possible final states, resolved by level, and/or bundling, appropriate for the GCR modeling. It will cover elements applicable to astrophysics and magnetic fusion viz. He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, Ca, Ti, Cr, Fe, Ni, Zn, Kr, Mo and Xe. The first... 

Paramyxoviruses cause respiratory tract diseases such as croup and pneumonia, as well as measles and mumps. The envelope proteins of these viruses share some features in common with influenza and retroviruses. These similarities include a precursor protein that is cleaved into two fragments, the second of which, called El, bears a fusion peptide at its amino terminus. In addition, peptides from the paramyxovirus FI proteins assemble into stable helical bundles resembling HIV gp41 and influenza HA2 (Baker et al, 1999 Lawless-Delmedico et al, 2000 Zhao et al, 2000). The paramyxovirus F protein differs from influenza HA and retroviral TM... 

Elegant studies have been carried out to investigate the structural and biochemical aspects of virus-cell fusion. Although influenza virus hemagglutinin and human immunodeficiency virus (HIV) gpl20 have been the best-studied models, numerous examples of this class I type fusion mechanism have been described. These experiments have shown that fusion is initiated by the formation of a trimeric coiled-coil helix adjacent to the fusion peptide on the virus exterior, the insertion of this fusion peptide into the host cell membrane, and the subsequent formation of a six-helix bundle (Skehel and Wiley, 1998). 

SNARE proteins assemble into a heterotrimeric SNARE complex (or core complex). The crystal structure of the core complex revealed a bundle of four a-helices, one each contributed by synaptobrevin and syntaxin and two contributed by SNAP-25 (28). The process of SNARE complex assembly proceeds from N-terminal to C-terminal direction in what is commonly referred to as a zippering action, which brings the C-terminal membrane anchors of the transSNAREs together (29). This action is proposed to force the closely apposed membranes together to initiate fusion. 

It is now well established that in vivo efficient membrane fusion requires the interaction of small cytoplasmically exposed membrane proteins called soluble N-ethylmaleimide sensitive factor (NSF) attachment receptors (SNAREs) (Sollner et al., 1993). For synaptic vesicle exocytosis, the relevant SNAREs are synaptobrevin/ vesicle-associated membrane protein (VAMP) 1 and 2, syntaxin 1, and synaptosome-associ-ated protein of 25,000 daltons (SNAP-25). Synaptobrevins/ VAMPs are localized primarily on synaptic vesicles, while syntaxin and SNAP-25 are localized primarily on the plasma membrane. Fusion is driven by the progressive zippering of vesicle and plasma membrane SNAREs forming a four-helix bundle (Sutton et al., 1998). Although many other proteins appear to play critical roles in synaptic vesicle exocytosis, it seems likely that SNAREs are the minimal machinery required for fusion (Weber et al., 1998). Once assembled, SNARE complexes are disassembled by NSF, which functions in conjunction with SNAP proteins. 

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