The dynamic microfilament system : cellular organization of actin and profilin and their association with cell signalling

Sammanfattning: Active shape changes and movement of whole cells, and translocations within cells are essential to life. For instance, during embryogenesis cell migration is a central theme. Wound healing depends on increased and directed motile activity of several cell types. Platelets change their shape dramatically during blood clotting and restoration of blood flow. Cells of the immune system extravasate (leave blood vessels) and migrate to sites of infection, where they carry out their defense reactions etc. Furthermore, increased motile activity is one of the hallmarks of malignant tumor cells.All these movements are carried out by a system of muscle proteins. They are present in all our cells, in fact in all eukaryotic cells. The proteins actin and myosin play central roles in this system. They form superstructures inside the cell, converting chemical energy into movements. In non-muscle cells, they are part of a highly dynamic system called the microfilament system, where the basic component are filaments formed from actin. Microfilaments are particularly concentrated beneath the plasma membrane, where they are linked, directly or indirectly, to transmembrane proteins - receptors, adhesion proteins, and ion channels. The dynamic activity of the microfilament system is based on the ability the cell has to change the system, both with respect to organization and activity, and it does so in response to the interactions that it has with the immediate surroundings.The protein profilin is one of the crucial components in the regulation of the microfilament system. It binds to actin and controls its polymerizability. In addition to that, it appears to be an important link between the transmembrane signalling and the mechanisms that polymerize actin into filaments.This thesis deals with the role of profilin in vivo. It reports on the localization of profilin in cells and describes the effects of microinjecting normal and mutant profilins, and a chemically crosslinked profilin:actin complex into cells. The results strengthen the view that profilin plays a role in the activation of the microfilament system, and provides evidence that the profilin:actin complex is the precursor for filament formation and that dissociation of the complex is a crucial step in the polymerization reaction. The thesis also describes effects on the microfilament system by microinjection of antibodies against two isoforms of PI3-kinase, an enzyme that is involved in the metabolism of a particular type of phospholipid present in the plasma membrane and linked to the control of the microfilament system.