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By the beginning of 1980s the questions concerning the mechanism of neurosecretion, the mode of neurotoxin action on this process and the mechanism of protein insertion into biological membranes were the principal directions of studies. Two views on the process of insertion of membrane proteins into, and transport across the biological membranes have been proposed. One hypothesis on membrane biogenesis describes the mechanism of incorporation of protein molecules into phospholipid matrix as a co-translational process. It starts with the synthesis of a hydrophobic N-terminal signal sequence of the protein destined to span the bilayer. In this case the energy of elongation was supposed to be utilized for insertion of the protein into hydrophobic core of the bilayer. Using the model system of co-translational translocation that consisted of liposomes and cell-free translational system including wheat germ extract and poly(A) RNA obtained from the mammary gland it has been studied whether the liposomes could serve as acceptor of synthesized secretory proteins. It is shown that in the presence of casein mRNA the 14-C-labeled product of translation is accumulated inside liposomes. When mRNA for globin, a nonsecretory protein, is translated in this cell-free system, 14-C-label is not found in the internal volume of liposomes. The polypeptides extracted from liposomes after their incubation in the system of casein mRNA translation interact specifically with anti-casein antibodies (D. I. Balkov, A. V. El'skaya, V. K. Lishko et al., 1988). These data provide the evidence that the transfer of synthesizing casein through bilayer lipid membrane does not require a specific receptor. Insertion and transfer of this protein occur co-translationally, due to the interaction of "signal peptide" with membrane lipids. The second model of the assembly of protein into membrane supposes an ability of hydrophilic proteins to cooperate with the phospholipid bilayer. As a result of such cooperation the polypeptide changes its conformation adequately to the hydrophilic environment and is integrated with the bilayer. This second model suggests the existence of water soluble precursors of membrane proteins localized in the cell cytoplasm. The last idea was studied in experiment with a specific hydrophilic protein from the cytoplasmic fraction of excitable tissues. Tetrodotoxin-sensitive (TTX-sensitive) structures have been found in soluble fractions of the brain, heart and skeletal muscle homogenates (V. K. Lishko, M. K. Malysheva, A. V. Stefanov, 1977). Initially we prepared proteoliposomes by sonication of a phospholipid suspension with the supernatants of tissue homogenates.(ABSTRACT TRUNCATED AT 250 WORDS)
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