Actin is a 42 000 Mw globular protein which is known to play an essential role in muscle contraction and which is suggested to have an equally important function in non-muscle cell motility. In isotonic solutions actin polymerizes to form double-stranded helixes with lengths of several microns. These filaments are the functional form of actin in the muscle sarcomere. Also in non-muscle cells actin filaments, originally described as microfilaments, seem to be a functional unit in the motility apparatus.

From non-muscle tissues actin can be purified in complex with a 15 000 Mw protein called profilin. The complex is called profilactin and is suggested to be the precursor to actin filaments. In the "Induced filament formation model of cell motility" actin is polymerized and depolymerized in a reversible manner with profilin acting as the regulator of actin polymerizability.

There is evidence for the co-existence of a monomeric and a polymerized form of actin in several non-muscle cell types. In blood platelets there is evidence for a conversion of actin from profilactin to filamentous actin as a result of stimulation. If profilactin is the precursor to actin filaments the dissociation of profilin from actin would be the primary event in the formation of microfilaments.

The work presented in this thesis demonstrates the decisive importance of the C-terminal part of actin for its interaction with profilin. In the case of profilin the C-terminus is suggested to have a considerable influence on the stability of the molecule although it seems to be of relatively little importance for the formation of the profilactin complex. The results presented here also indicate hydrophobic properties of profilin induced by the oxidation of the molecule which might be of significance to its functioning in vivo.