The interphase nucleus compartmentalizes its components to give rise to a highly organized and tightly controlled environment. Individual chromosomes occupy discrete areas, termed "chromosome territories," that are separated from each other by a channel called the "interchromosomal domain" (reviewed in Lamond and Earnshaw 1998). Actively transcribed genes tend to be at the periphery of chromosomal territories, whereas newly made RNA transcripts localize into the interchromosomal domain, where they can undergo further processing and transport. Movement within the nucleus (Ferreira et al. 1997) may permit chromosomes to enter "factories" that contain all the necessary enzymatic machinery for replication (reviewed in Jackson 1995).Of the many discrete domains identified throughout the nucleus, the largest are nucleoli, sites of ribosomal RNA synthesis and processing, and sites of preribosomal particle assembly (reviewed in Scheer and Weisenberger 1994). Other subnuclear bodies that appear as punctate structures under immunofluorescence (IF) microscopy include various dynamic structures involved in the maintenance and replication of DNA and RNA synthesis, processing, and transport (reviewed in Nickerson et al. 1995): replication foci, transcript foci, speckled domains, coiled bodies, gems, and promyelocytic leukemia protein (PML) nuclear bodies. Spliceosomal small nuclear (sn) ribonucleoprotein (RNP) components and a subset of non-snRNP splicing factors can be found concentrated in discrete subnuclear domains called "coiled bodies" (Matera and Frey 1998 [in this issue]). It is becoming increasingly apparent that the nucleus has an organization and contains a number of discrete macromolecular domains that coordinate a variety of nuclear processes.

Nuclear magnetic relaxation rates for water protons in aqueous palmitoyloleoylphosphatidylcholine vesicle suspensions containing different nitroxide free radical spin labels are reported as a function of magnetic field strength corresponding to proton Larmor frequencies from 10 kHz to 30 MHz. Under these conditions the water proton relaxation rate is determined by the magnetic coupling between the water protons and the paramagnetic nitroxide fixed on the phospholipid. This coupling is made time-dependent by the relative translational motion of the water proton spins past the nitroxide radical. Using theories developed by Freed and others, we interpret the NMR relaxation data in terms of localized water translational motion and find that the translational diffusion constant for water within approximately 10 A of the phospholipid surface is 6 x 10(-10) m2 s(-1) at 298 K. Similar results are obtained for three different nitroxide labels positioned at different points on the lipid. The diffusion is a thermally activated process with an activation energy only slightly higher than that for bulk water.

1H nuclear magnetic relaxation dispersion experiments show remarkable differences between water and acetone in contact with microporous glass surfaces containing trace paramagnetic impurities. Analyzed with surface relaxation theory on a model porous system, the data obtained for water show that proton surface diffusion limited by chemical exchange with the bulk phase permits long-range effectively one-dimensional exploration along the pores. This magnetic-field dependence coupled with the anomalous temperature dependence of the relaxation rates permits a direct interpretation in terms of the proton translational diffusion coefficient at the surface of the pores. A universal rescaling applied to these data collected for different pore sizes and on a large variety of frequencies and temperatures, supports this interpretation. The analysis demonstrates that acetone diffuses more slowly, which increases the apparent confinement and results in a two-dimensional model for the molecular dynamics close to surface relaxation sinks. Surface-enhanced water proton diffusion, however, permits the proton to explore a greater spatial extent of the pore, which results in an apparent one-dimensional model for the diffusive motions of the water that dominate nuclear spin relaxation.

Ubiquitin modification is a well established way of regulating protein levels and activities. Modification by related ubiquitin-like proteins is turning out to have a diverse range of interesting cellular functions.

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