Progress in Molecular and Subcellular Biology

Association of DNA with Nuclear Matrix.- A. Introduction.- B. MAR Elements and Their Properties.- C. Roles of MAR Elements in DNA Packaging and DNA Replication.- References.- Nuclear Import in Vitro.- A. Overview of Nuclear Import.- I. Nuclear Import Occurs Through the Nuclear Pore.- II. The Structure and Composition of the Nuclear Pore.- III. Functional Properties of the Nuclear Pore.- IV. Signals for Nuclear Accumulation.- V. The Effect of Multiple Nuclear Targeting Signals in a Protein.- VI. Is There a Role for Intranuclear Binding in Nuclear Import?.- VII. Nuclear Import is an Active Process. Evidence for a Signal Sequence Receptor.- VIII. Is the Signal Sequence Receptor Heterogeneous?.- IX. Regulation of Nuclear Import.- 1. Changes in Nuclear/Cytoplasmic Localization During Embryogenesis.- 2. Nuclear Import of snRNP Particles.- 3. Nuclear Exclusion of Transcription and Replication Factors: a Regulatory Mechanism?.- 4. Reversible Nucleocytoplasmic Movement.- 5. Nuclear Import of Hormone Receptors: Hormone-Dependent?.- B. In Vitro Systems for Studying Nuclear Import.- I. In Vitro Systems Based on Xenopus Egg Extracts.- 1. Rationale for the Use of Egg Extracts.- 2. Nuclear Import Activity in Egg Extracts is Authentic.- 3. Nuclear Import is Inhibited by the Lectin, Wheat Germ Agglutinin. Are Nuclear Pore Glycoproteins Involved in the Transport Mechanism?.- 4. Nuclear Import is Separable Experimentally into Two Steps: Signal-Mediated Binding and ATP-Dependent Translocation.- II. In Vitro Systems Based on Isolated Nuclei in Buffers.- 1. Heparin-Extracted Nuclei.- 2. Systems Involving Whole Isolated Nuclei in Defined Buffers.- III. Passive Influx into Isolated Nuclei.- C. Conclusion.- References.- Cytoplasmic Assembly and Nuclear Transport of the snRNP Particles.- A. Introduction.- B. Maturation of the snRNAs.- I. snRNAs Appear Transiently in the Cytoplasm.- II. 3’ End Processing of snRNAs.- III. Nucleotide Modification Including the 5’ Cap.- C. snRNP Particle Assembly.- I. snRNP Proteins.- II. snRNP Core Protein Assembly.- III. Relationship of B, B’, and N snRNP Core Proteins.- IV. U1 and U2 Specific Proteins.- V. Independent Synthesis and Assembly of snRNP Proteins in Xenopus Oocytes.- VI. In Vitro Assembly of snRNP Particles.- VII. U6 snRNP.- D. Nuclear Accumulation of snRNP Particles.- I. Interphase.- 1. snRNP Core Particles.- 2. snRNP Specific Proteins.- II. Mitosis.- E. Summary and Perspectives.- References.- The Centrosome: Recent Advances on Structure and Functions.- A. Definitions.- B. Isolation of Centrosomes from Somatic Cells.- C. The Structure of Isolated Centrosomes.- D. Centrosomal Proteins.- E. The Centrosome and the Nucleation of Microtubules.- F. The Centrosome Cycle in the Cell Cycle.- G. Centrosome Continuity.- H. Centrosome and the Spindle Formation.- I. Centrosome and the Spatial Organization of Microtubules in Terminally Differentiated Cells.- J. Centrosome and Cell Polarity/Movement.- K. Conclusion.- References.- Role of Nonsense, Frameshift, and Missense Suppressor tRNAs in Mammalian Cells.- A. Introduction.- B. Suppression of Nonsense Codons.- I. Naturally Occurring Nonsense Suppressor tRNAs.- 1. Amber Suppressor tRNAs.- 2. Ochre Suppressor tRNAs.- 3. Opal Suppressor tRNAs.- II. Assays for Nonsense Suppressor tRNAs and Nonsense Mutations.- III. Introduction of Nonsense Suppressor tRNA Genes into Intact Cells.- IV. Other Considerations.- C. Ribosomal Frameshifting.- I. Ribosomal Frameshifting in Retroviruses.- II. tRNAs Involved in Frameshifting.- D. Missense Suppression and Misrecognition of Genetic Codewords.- E. Conclusion.- References.- UAG Suppressor Glutamine tRNA in Uninfected and Retrovirus-Infected Mammalian Cells.- A. Introduction.- B. Isolation and Sequence Analysis of Glutamine tRNA from Mammalian Cells.- C. Analysis of Suppressor Activity of Mammalian Glutamine tRNA.- D. Selective Increase of Suppressor Glutamine tRNA in Retrovirus-Infected Cells.- E. Influence of Increased Amount of Suppressor tRNA on Translation Reaction of Cellular mRNA.- F. Discussion.- References.- Essential Genes for Development of Dictyostelium.- A. Introduction.- B. Genetic Analysis in Dictyostelium.- C. Temporal Sequence of Differentiations.- D. Initiation of Development.- E. Stage 1: Chemotaxis.- F. cAMP Regulation of Transcription and Chemotaxis.- G. Stage 2: Integration.- H. Stage 3: Divergence.- I. Stage 4: Culmination.- J. Dependent Sequence.- References.