Seibutsu Butsuri

Among a great number of exotoxins produced by pathogenic bacteria, only several toxins including diphtheria toxin and chnolera toxin have been shown how to exert their toxic effects on susceptible cells.
Diphtheria toxin that contains two polypeptides, fragment A and B, binds to an unidentified receptor on cell surface by interaction of fragment B with the receptor. Then fragment A released by unknown mechanism into cytoplasm catalyzes ADP-ribosylation of eukaryotic elongation factor 2 (EF2), which results in ceasing of protein synthnesis.
Cholera toxin, on the other hand, consists of single A subunit and several B subunits which are the binding sites of the toxin to a cell surface receptor, ganglioside G_Ml. Following release of subunit A through the membrane into cytoplasm induces the activation of adenylate cyclase through inhibition of hydrolysis of GTP at the regulatory site of adenylate cyclase system. Subunit A is believed to be an enzyme which ADP-ribosylates one of the components of adenylate cyclase.
Pseudomonas aeruginosa exotoxin and Escherichia coli heat-labile enterotoxin as well as diphtheria toxin and cholera toxin require NAD in thein action. P. aeruginosa toxin inhibits protein synthesis by ADP-ribosylation of EF2 and E. coli toxin activates adenylate cyclase.

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Movement is one of the fundamental properties of living cells. It is expressed in such diverse cellular activities in animal and plant cells as cytoplasmic streaming, amoeboid movement, phagocytosis, morphogenetic movement, cytokinesis, mitosis, secretory processes and probably the regulation of the topological distribution of membrane proteins as well.
Recently proteins which closely resemble the major contractile proteins of muscle, actin and myosin, have found in most types of eukaryotic cells and even in prokaryotic cells. The universal occurence of these proteins suggests that they provide the basis of a general mechanism for producing cellular movement, of which muscle contraction is a specialized example.
In general the nonmuscle actins (cytoplasmic actins) resemble the muscle actin, and this similarity was mainly emphasized in early researches. However, more extensive biochemical studies have increasingly revealed some differences, especially in microheterogenity among actins. Another significant focus of recent researches concerns the ability of nonmuscle actins to form supramolecular assemblies or higher aggregates corresponding to the microfilament bundles or stress fibers seen in microscopy. These studies suggest that actin may play a cytoskeletal as well as a contractile role in nonmuscle cells.
In this review we have focused our attension to emphasize recent advances in the biochemical understanding of the nonmuscle actin. We discuss the properties of the purified actin and the morphological distribution of actin filaments and its properties. Most of the papers discussed here were published after 1974.

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