Proteins depolymerization

In cooperation with many kinds of actin-binding proteins, the G-F transformation of actin generates translational movement of a bacterial cell in a host cell, as in Figure 10 [46], Anchoring and nucleation proteins, depolymerizing proteins, and cross-linking proteins work to make possible a fast cycle of actin molecules from one end of F-actin to the other end, that is, tread-milling. This system has been artificially reconstructed in vitro [47]. 

Rabbit Muscle Actin. The monomeric G form of actin carries an ATP firmly bound, which is hydrolyzed in a stoichiometric reaction to ADP during polymerization to the fibrillar F form under the influence of inorganic cations, especially Mg. By removal of the triggering cations, and displacement of the ADP by fresh ATP, the protein depolymerized again to the G form. 

The membrane tubules and lamellae of the endoplasmic reticulum (ER) are extended in the cell with the use of MTs and actin filaments. Kinesin motors are required for stretching out the ER, whereas depolymerization of microtubules causes the retraction of the ER to the cell centre in an actin-dependent manner. Newly synthesized proteins in the ER are moved by dynein motors along MTs to the Golgi complex (GC), where they are modified and packaged. The resulting vesicles move along the MTs to the cell periphery transported by kinesin motors. MTs determine the shape and the position also of the GC. Their depolymerization causes the fragmentation and dispersal of the GC. Dynein motors are required to rebuild the GC. 

Partial etherification of the beech wood MGX with p-carboxybenzyl bromide in aqueous alkali yielded fully water-soluble xylan ethers with DS up to 0.25 without significant depolymerization the Mw determined by sedimentation velocity was 27 000 g/mol [400,401]. By combination of endo- 6-xylanase digestion and various ID- and 2D-NMR techniques, the distribution of the substituents was suggested to be blockwise rather than uniform. The derivatives exhibited remarkable emulsifying and protein foam-stabilizing activi-... 

Microtubule-associated proteins bind to microtubules in vivo and subserve a number of functions including the promotion of microtubule assembly and bundling, chemomechanical force generation, and the attachment of microtubules to transport vesicles and organelles (Olmsted, 1986). Tubulin purified from brain tissue by repeated polymerization-depolymerization contains up to 20% MAPs. The latter can be dissociated from tubulin by ion-exchange chromatography. The MAPs from brain can be resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). 

The ER has a reticular morphology which provides a large surface area, which presumably is required for the synthesis and transport of proteins and lipids and for the storage of calcium. The ER is associated with microtubules, and the two are highly interdependent structures. Terasaki et al. (1986) found that when microtubules in the cell are depolymerized by colchicine, the ER network slowly retracts toward the center of the cell. If the microtubules are repolymerized, the ER network is restored to its original morphology, thereby suggesting that the MTs participate in the formation and maintenance of the ER. 

Phase 3 (5 to 7.5 g/l) At these concentrations, a drop in PG activity is observed. This phenomenom is reprocible and can be attributed to pectin-protein interactions. These interactions only occur between specific pectin-protein couples after a first depolymerization action of PG Depolymerized pectins associate with proteins to yield aggregates which can easily sediment. Their removal during medium centrifugation would explain the lowering in PG activity in the reaction medium over this polygalacturonic acid concentration range. 

Representative condensation polymers are listed in Table I. The list is by no means exhaustive, but it serves to indicate the variety of condensation reactions which may be employed in the synthesis of polymers. Cellulose and proteins, although their syntheses have not been accomplished by condensation polymerization in the laboratory, nevertheless are included within the definition of condensation polymers on the ground that they can be degraded, hydrolytically, to monomers differing from the structural units by the addition of the elements of a molecule of water. This is denoted by the direction of the arrows in the table, indicating depolymerization. 

The answer is d. (Hardman, p 1258. Katzung, pp 935-9.36.) Vinblastine binds to tubulin and blocks the protein from polymerizing to microtubules. The drug-tubulin complex binds to the developing microtubule, resulting in inhibition of microtubule assembly and subsequent depolymerization. 

Microtubules have a key role in mitosis and cell-proliferation. They are dynamic assemblies of heterodimers of a and f3 tubullin. In the cell-reproduction cascade tubulin polymerizes fast and subsequently depolymerizes. Tubulin dimers are unusual guanyl nucleotide binding (G) proteins, which bind GTP reversibly at a site in the (3-tubulin. GTP irreversibly hydrolyzes to GDP during polymerization. 

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