Ingrid Zwaenepoel
The ability of cells to adopt a variety of shapes and to carry out coordinated and directed movements depends on the cytoskeleton, a complex network of filamentous proteins that extends throughout the cell. Three main protein systems constitute the cytoskeleton: actin filaments, microtubules and intermediate filaments.
Actin is encoded by a gene family producing six different isoforms in vertebrates. Four isoactins are synthesized in different muscle cells, whereas two isoforms are preponderant in non-muscle cells: beta and gamma- cytoplasmic actins. However, to date little is known about the pattern and function of these two isoforms We are interested in better understanding how expression of both isoforms is regulated at different steps (transcription: i.e., the conversion of DNA to RNA; translation: i.e., the conversion of RNA to protein) and whether each isoform displays a preferential localization and exerts specialized functions.
We have compared the distribution of both proteins in different cell types such as in epithelial cells that line the outside of nearly all organs and in fibroblasts that are the most common cells of connective tissue. We show for the first time that beta andgamma- actins organize in a complete different way: they are localized at different sites within the cell and they have different structural organization. beta-actin is preferentially localized in contractile bundles (in cables at intercellular contacts between epithelial cells and in stress fibres in fibroblasts) whereas gamma-CYA is mainly organized as a network in cortical and motile parts of cells (i.e. lamellipodia) Our results also indicate that beta and gamma- actins fulfill distinct functions: beta-actin may rather play a role in contraction and cell attachment whereas gamma-actin would participate mainly in cell flexibility and movement. In the future, these findings may help to better understanding pathological situations, such as tumour invasion and metastasis (characterizing cancer malignancy), in which cell movements are aberrant.
