Intranuclear targeting of DNA replication factors.

Mammalian nuclei are highly organized into functional compartments. Major nuclear processes like DNA replication and RNA processing take place in distinct foci. These microscopically visible foci are formed by the assembly of, for example, DNA replication factors and associated proteins into megadalton complexes often referred to as protein machines or factories. Thus far, two proteins, DNA ligase I and DNA methyltransferase (DNA MTase), have been analyzed in greater detail. In both cases, the assembly process appears to be controlled by distinct targeting sequences that were attached to the catalytic protein core in the course of evolution and mediate the association with replication factories in mammalian cells. The dynamics of these nuclear structures throughout the cell cycle are analyzed using green fluorescent protein (GFP). Further studies are needed to elucidate the architecture, regulation, and role of these subnuclear structures.