Profilin From the Cell Edge into the Nucleus

University dissertation from Stockholm : Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University

Abstract: Internal and external signaling dependent changes in cell behavior are directly linked to force-generating remodeling of the actin microfilament system which is juxtaposed to the inside of the plasma membrane. This dynamic filament system is involved in many processes in the cytoplasm and the nucleus of eukaryotic cells. This thesis studies profilin, a regulator of actin filament dynamics which functions during incorporation of new actin molecules at growing filament ends at the cell periphery. Profilin is also present in the nucleus but its function is less well understood in this compartment. Here I present results concerning profilin and the activity of the transcription factor SRF, which is known to control the expression of actin and many actin-binding proteins in a process requiring the MRTF-A co-factor. MRTF-A binds monomeric actin and is released upon receptor mediated actin polymerization. Depletion of the two profilin isoforms I and IIa reduced MRTF-A/SRF-dependent transcription, most likely since the lack of profilin enable more MRTF-A to bind actin monomers and thereby prevent SRF-transcription. Interestingly profilin depletion also seemed to affect general transcription in the two cell lines investigated. In a separate study, a close connection between profilin, and possibly also profilin:actin, with microtubules was revealed. Microtubules are important for intracellular trafficking of vesicles as well as directional cell migration and the observation made here suggests the existence of a microtubule-associated platform for actin filaments formation. In congruence, the microtubule-associated actin nucleation promoting factor WHAMM was found to interact with profilin. Finally, the intracellular distribution of profilin was investigated by fluorescence microscopy using different peptide specific antibodies. Since these antibodies showed unique but varying results our work emphasizes common problems connected with this technique.