|
General Comment |
Upstream of YAP/////
Myocardin/MKL family of SRF coactivators: key regulators of immediate early and muscle specific gene expression. Cen B et al. (2004) Myocardin, megakaryoblastic leukemia-1 (MKL1), and MKL2 belong to a newly defined family of transcriptional coactivators. All three family members bind to serum response factor (SRF) and strongly activate transcription from promoters with SRF binding sites. SRF is required for the serum induction of immediate early genes such as c-fos and for the expression of many muscle specific genes. Consistent with a role in muscle specific gene expression, myocardin is specifically expressed in cardiac and smooth muscle cells while MKL1 and 2 are broadly expressed. Myocardin has particularly been shown to be required for smooth muscle development while MKL1/2 are required for the RhoA signaling pathway for induction of immediate early genes. SRF can be activated by at least two families of coactivators, p62TCF and myocardin/MKL. These factors bind to the same region of SRF such that their binding is mutually exclusive. This provides one mechanism of regulation of SRF target genes by pathways that differentially activate the coactivators. The RhoA pathway appears to activate MKL1 by altering MKL1's binding to actin and causing MKL1's translocation from the cytoplasm to the nucleus. However, this mechanism of activation of the myocardin/MKL family has not been observed in all cell types such that other regulatory mechanism(s) likely exist. In particular, rapid serum inducible phosphorylation of MKL1 was observed. The regulation of this coactivator family is key to understanding how SRF target genes are activated during muscle cell differentiation or growth factor induced cell proliferation.//////////////////
A nuclear MAL-function links Rho to SRF. Settleman J et al. (2003) The activation of SRF-mediated transcription in response to Rho GTPase-induced actin assembly involves the ability of SRF to sense changes in the cellular level of actin monomers. An article in the May 2 issue of Cell identifies the transcription factor MAL as an actin binding protein that functions as an SRF coactivator and whose translocation from cytoplasm to nucleus depends on its dissociation from actin monomers (Miralles et al., 2003).//////////////////
|