Taehoon Chun

The interaction between E‐tropomodulin and thymosin β‐10 rescues tumor cells from thymosin β‐10 mediated apoptosis by restoring actin architecture

Thymosin β‐10 (TB10) is a small G‐actin binding protein that induces depolymerization of intracellular F‐actin pools by sequestering actin monomers. Previously, we demonstrated that overexpression of TB10 in ovarian tumor cells increased the rate of cell death. As an initial step to define molecular mechanism of TB10‐dependent apoptotic process in ovarian tumor cells, we searched a human ovary cDNA library for a novel TB10 binding protein using a yeast two‐hybrid system. The selected protein was human E‐tropomodulin (E‐Tmod), another component of the actin binding proteins. Subsequently, two interacting protein components were determined quantitatively. Results showed that the full‐length TB10 is required to bind with E‐Tmod, and the TB10 binding site on E‐Tmod partially overlaps with the actin binding site on E‐Tmod. Moreover, introduction of E‐Tmod cDNA into a tumor cell line reversed TB10 mediated apoptosis and restored actin architectures. These results may suggest that TB10 regulates apoptotic homeostasis by not only just binding to actin but also competing or blocking the protein complex formation of E‐Tmod with actin.

The polycomb group gene product Mel‐18 interacts with cyclin D2 and modulates its activity

Considerable evidence supports the view that D‐type cyclins play a role in G1‐S progression. We found that cyclin D2 directly interacts with Mel‐18, one of the polycomb group gene products in a yeast two hybrid screen. Further, we have determined the binding domains that are required for interaction between cyclin D2 and Mel‐18. The proline/serine‐rich domain (P/S domain) of Mel‐18 is required to interact with cyclin D2, and the N‐terminal region of cyclin D2 is necessary to interact with Mel‐18. A co‐localization study shows that cyclin D2 and Mel‐18 interact within the nucleus. To determine whether Mel‐18 affects cyclin D2 activity, we blocked Mel‐18 expression using an anti‐sense strand system in cyclin D2 over‐expressing cells. The results indicate that cells with reduced Mel‐18 expression levels show more proliferative activity than the controls. These findings are the first report that Mel‐18 directly interacts with cyclin D2 and may inhibit cyclin D2 activity.