Drug-induced alterations of cytokeratin organization in cultured epithelial cells

The distribution of keratin intermediate filaments, previously considered static in organization and imperturbable by conventional drugs used to alter the structure and organization of the cytoskeleton, can be altered significantly by treatment with colchicine and cytochalasin D. The loss of microfilaments and microtubules converts the keratin cytoskeleton from a branching, even distribution to a series of starlike structures whose filaments are maintained by multiple membrane attachment sites. These findings provide a means for manipulating cytokeratin organization to investigate the role of keratins in cytoskeletal structure and function.     

Actin polymerization and ATP hydrolysis

F-actin is the major component of muscle thin filaments and, more generally, of the microfilaments of the dynamic, multifunctional cytoskeletal systems of nonmuscle eukaryotic cells. Polymeric F-actin is formed by reversible noncovalent self-association of monomeric G-actin. To understand the dynamics of microfilament systems in cells, the dynamics of polymerization of pure actin must be understood. The following model has emerged from recent work. During the polymerization process, adenosine 5'-triphosphate (ATP) that is bound to G-actin is hydrolyzed to adenosine 5'-diphosphate (ADP) that is bound to F-actin. The hydrolysis reaction occurs on the F-actin subsequent to the polymerization reaction in two steps: cleavage of ATP followed by the slower release of inorganic phosphate (Pi). As a result, at high rates of filament growth a transient cap of ATP-actin subunits exists at the ends of elongating filaments, and at steady state a stabilizing cap of ADP.Pi-actin subunits exists at the barbed ends of filaments. Cleavage of ATP results in a highly stable filament with bound ADP.Pi, and release of Pi destabilizes the filament. Thus these two steps of the hydrolytic reaction provide potential mechanisms for regulating the monomer-polymer transition.     

The cytoskeletal protein vinculin contains transformation-sensitive, covalently bound lipid

Vinculin, which is associated with the cytoskeleton of many cells, has been suggested as a possible linker between microfilament bundles and the plasma membrane. Here it will be shown that fatty acid is covalently attached to vinculin in vivo. Furthermore, in chicken embryo fibroblasts infected with a temperature-sensitive mutant of Rous sarcoma virus, tsNY68, the acylation of vinculin at the permissive temperature was less than one-third that at the nonpermissive temperature. Thus, the covalent binding of lipid to vinculin is a transformation-sensitive event. The covalent modification of vinculin by lipids could be directly or indirectly involved in its reversible association with membranes. This modification may also provide a mechanism to alter the organization of vinculin within cells and thereby play a regulatory role in anchoring or stabilizing microfilament bundles at plasma membranes.     

Muscle relaxation: evidence for an intrafibrillar restoring force in vertebrate striated muscle

Observations of contracting muscle fibrils in cultured cells indicate that the force which restores the resting length of the sarcomere comes from the contractile elements themselves and not from external elasticity, as is now generally accepted. In light of biochemical studies on the contraction-relaxation cycle, it is postulated that the elongating force is one of internal elasticity in the sarcomere, which arises during contraction from the distortion of bonds between filaments and/or structural proteins. This mechanism of restoration may serve to establish optimal sarcomere length for production of maximum contractile force, and in cardiac muscle this mechanism may be a factor in ventricular filling.     

Pancreatic beta-cell web: its possible role in insulin secretion

A cortical band of fine microfilaments is consistently observed in the beta cells of the rat pancreas. Alteration of this cell web by cytochalasin B is associated with an enhancement of glucose-induced secretion of insulin by isolated islets. The microfilamentous web of the beta cell may play an important role in the emiocytosis of insulin secretory granules, by controlling their access to the cell membrane.     

Macromolecular architecture in eukaryotic cells visualized by cryoelectron tomography

Electron tomography of vitrified cells is a noninvasive three-dimensional imaging technique that opens up new vistas for exploring the supramolecular organization of the cytoplasm. We applied this technique to Dictyostelium cells, focusing on the actin cytoskeleton. In actin networks reconstructed without prior removal of membranes or extraction of soluble proteins, the cross-linking of individual microfilaments, their branching angles, and membrane attachment sites can be analyzed. At a resolution of 5 to 6 nanometers, single macromolecules with distinct shapes, such as the 26S proteasome, can be identified in an unperturbed cellular environment.     

Actin network architecture can determine myosin motor activity

The organization of actin filaments into higher-ordered structures governs eukaryotic cell shape and movement. Global actin network size and architecture are maintained in a dynamic steady state through regulated assembly and disassembly. Here, we used experimentally defined actin structures in vitro to investigate how the activity of myosin motors depends on network architecture. Direct visualization of filaments revealed myosin-induced actin network deformation. During this reorganization, myosins selectively contracted and disassembled antiparallel actin structures, while parallel actin bundles remained unaffected. The local distribution of nucleation sites and the resulting orientation of actin filaments appeared to regulate the scalability of the contraction process. This "orientation selection" mechanism for selective contraction and disassembly suggests how the dynamics of the cellular actin cytoskeleton can be spatially controlled by actomyosin contractility.     

乙酰胆碱和微丝、微管抑制剂对大葱细胞 内含物再分配的影响

以大葱为材料,采用叶片环剥方法,分别用乙酰胆碱(Ach)、细胞松弛素B(CB)和甲酰氨草磷(APM)对裸露维管束处理,以叶片中N、P和K为指标,研究了这三种药剂对细胞内含物再分配的影响。结果表明,低浓度Ach促进细胞内含物的运输;CB和APM均有效地减弱了细胞内含物的运输。初步从维管束水平上证实,维管束中原生质参与韧皮部运输。     

烟草BY-2细胞和蚕豆叶肉细胞原生质体微丝骨架及latrunculin B处理对其分布的影响

用酶解法分别制备烟草BY-2悬浮培养细胞和蚕豆叶肉细胞的原生质体,应用激光共聚焦显微镜观察经TRITC-鬼笔环肽荧光染色的2种原生质体的微丝骨架空间分布,以及观察用10和20μmol·L-1微丝抑制剂latrunculin B分别处理2种原生质体后对其分布的影响.结果表明:激光共聚焦显微镜清楚地显示出2种细胞原生质体中微丝精细排列的均匀网络结构;共聚焦显微成像显示了随latrunculin B处理时间的延长微丝解聚和微丝骨架分布的动态过程.建立原生质体制备、微丝骨架荧光染色、latrunculin B处理及激光共聚焦显微观察的实验体系为进一步研究植物微丝骨架的功能奠定了基础.     

Perineurium: evidence for contractile elements

Electron-microscopic study of mouse sciatic nerve reveals that perineurial cells contain filaments and associated opaque regions similar to those observed in smooth muscle. This finding is consistent with obsevations Which suggest that nerve might have a contractile property. In addition to their function in maintaining the connective tissue stroma of perineurium, as well as being a selective diffusion barrier, perineurial cells may serve the nerve in a contractile capacity.