Forced expression of tropomyosin 2 or 3 in v-Ki-ras-transformed fibroblasts results in distinct phenotypic effects.

Transformation of cells in tissue culture results in a variety of cellular changes including alterations in cell growth, adhesiveness, motility, morphology, and organization of the cytoskeleton. Morphological and cytoskeletal changes are perhaps the most readily apparent features of transformed cells. Although a number of studies have documented a decrease in the expression of specific tropomyosin (TM) isoforms in transformed cells, it remains to be determined if the suppression of TM synthesis is essential in the establishment and maintenance of the transformed pheno-type. To address the roles of different TM isoforms in transformed cells we have examined the effects of expressing specific TM isoforms in transformed cells using a Kirsten virus-transformed cell line (ATCC NRK1569) as a model system. In contrast to normal fibroblasts, the NRK 1569 cells contain reduced levels of TM-1 and undetectable levels of TM-2 and TM-3. These cells have a rounded morphology and are devoid of stress fibers. Employing expression plasmids for TM-2 and TM-3, stable cell lines were established from the NRK 1569 cells that express these isoforms individually. We demonstrate that expression of TM-2 or TM-3 leads to increased cell spreading accompanied by the formation of identifiable microfilament bundles, as well as significant restoration of well-defined vinculin-containing focal adhesion plaques, although expression of each isoform exhibited distinct properties. In addition, cells expressing TM-2, but not TM-3, exhibited contact-inhibited cell growth and a requirement for serum.

Conversion of agonist site to metal-ion chelator site in the β2-adrenergic receptor

Previously metal-ion sites have been used as structural and functional probes in seven transmembrane receptors (7TM), but as yet all the engineered sites have been inactivating. Based on presumed agonist interaction points in transmembrane III (TM-III) and -VII of the β2-adrenergic receptor, in this paper we construct an activating metal-ion site between the amine-binding Asp-113 in TM-III—or a His residue introduced at this position—and a Cys residue substituted for Asn-312 in TM-VII. No increase in constitutive activity was observed in the mutant receptors. Signal transduction was activated in the mutant receptors not by normal catecholamine ligands but instead either by free zinc ions or by zinc or copper ions in complex with small hydrophobic metal-ion chelators. Chelation of the metal ions by small hydrophobic chelators such as phenanthroline or bipyridine protected the cells from the toxic effect of, for example Cu2+, and in several cases increased the affinity of the ions for the agonistic site. Wash-out experiments and structure–activity analysis indicated, that the high-affinity chelators and the metal ions bind and activate the mutant receptor as metal ion guided ligand complexes. Because of the well-understood binding geometry of the small metal ions, an important distance constraint has here been imposed between TM-III and -VII in the active, signaling conformation of 7TM receptors. It is suggested that atoxic metal-ion chelator complexes could possibly in the future be used as generic, pharmacologic tools to switch 7TM receptors with engineered metal-ion sites on or off at will.

Hybrid characteristics of oligonucleotides consisting of isonucleoside 2′,5′-anhydro-3′-deoxy-3′-(thymin-1-yl)-d-mannitol with different linkage modes

Oligonucleotides consisting of the isonucleoside repeating unit 2′,5′-anhydro-3′-deoxy-3′-(thymin-1-yl)-d-mannitol (4) were synthesized with the monomeric unit 4 incorporated into oligonucleotides as 1′→4′ linkage 4a (oligomer I) or 6′→4′ linkage 4b (oligomer II). The hybrid properties of the two oligonucleotides I and II with their complementary strands were investigated by thermal denaturation and CD spectra. Oligonucleotide I (4a) formed a stable duplex with d(A)14 with a slightly reduced Tm value of 36.6°C, relative to 38.2°C for the control duplex d(T)14/d(A)14, but oligomer II (4b) failed to hybridize with a DNA complementary single strand. The spectrum of the duplex oligomer I/d(A)14 showed a positive CD band at 217 nm and a negative CD band at 248 nm attributable to a B-like conformation. Molecular modeling showed that in the case of oligomer I the C6′ hydroxy group of each unit could be located in the groove area when hybridized to the DNA single strand, which might contribute additional hydrogen bonding to the stability of duplex formation.