THE NUCLEAR MATRIX - A CRITICAL-APPRAISAL

It is becoming increasingly clear that the cell nucleus is a highly st ructurized organelle. Because of its tight compartmentalization, it is generally believed that a framework must exist, responsible for maint aining such a spatial organization. Over the last twenty years many in vestigations have been devoted to identifying the nuclear framework. S tructures isolated by different techniques have been obtained in vitro and are variously referred to as nuclear matrix, nucleoskeleton or nu clear scaffold. Many different functions, such as DNA replication and repair, mRNA transcription, processing and transport have been describ ed to occur in close association with these structures. However, there is still much debate as to whether or not any of these preparations c orresponds to a nuclear framework that exists irt vivo. In this articl e we summarize the most commonly-used methods for obtaining preparatio ns of nuclear frameworks and we also stress the possible artifacts tha t can be created in vitro during the isolation procedures. Emphasis is placed also on the protein composition of the frameworks as well as o n some possible signalling functions that have been recently: describe d to occur in tight association with the nuclear matrix.