Transcription: Responding to depletion

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Richard Treisman's group have now found a missing link in Rho-actin signaling to serum response factor (SRF) — the myocardin-related protein MAL is a globular (G)-actin-responsive SRF coactivator.

Changes in actin dynamics don't just influence the cytoskeleton, they affect gene transcription too — Rho-family GTPase-induced actin polymerization activates the transcription factor serum response factor (SRF), which regulates immediate-early genes. Richard Treisman's group have now found a missing link in Rho–actin signalling to SRF — the myocardin-related protein MAL is a globular (G)-actin-responsive SRF coactivator.

The authors first showed that MAL relocalizes rapidly from the cytoplasm to the nucleus in response to serum or lysophosphatidic acid, both of which activate Rho and SRF, thereby implicating MAL in Rho–actin signalling. MAL interacted, preferentially as a dimer, with the DNA-binding domain of SRF, and potentiated SRF reporter activity in response to Rho activation. Inactivation of Rho by C3 transferase inhibited SRF reporter activation by MAL.

When the authors inhibited Rho, this prevented MAL nuclear accumulation. So, too, did expression of inactive forms of Dia1 or VASP — proteins that are normally involved in actin polymerization downstream of Rho — which have previously been reported to interfere with SRF activation. Overexpression of actin also inhibited MAL nuclear translocation. Conversely, expression of proteins, such as RhoA, that can promote actin polymerization and SRF activation induced MAL nuclear translocation. Furthermore, certain actin-binding drugs that stabilize filamentous (F)-actin and activate SRF can efficiently induce MAL nuclear translocation. Rho–actin signalling, MAL nuclear translocation and SRF activation therefore seem to be tightly correlated.

MAL proteins have several conserved sequence motifs, including three conserved amino-terminal RPEL motifs. The amino-terminal domain, in particular RPEL motifs 2 and 3, was required for nuclear accumulation of MAL in response to Rho signalling. Carboxy-terminal sequences and a Q-rich domain (the so-called Q-box) were implicated in a distinct function, possibly cytoplasmic retention or nuclear export, whereas two regions rich in basic residues — B1 and B2 — were shown to be required for serum-induced nuclear accumulation.

As MAL potentiated SRF activity, the authors investigated whether mutations that render MAL cytoplasmic interfered with SRF activation. Deletion of B1, or B1 and B2, specifically inhibited Rho–actin signalling at the level of MAL rather than SRF — genes regulated by SRF independently of Rho–actin signalling were unaffected. The authors then showed that MAL bound specifically — in a stimulus-specific manner — to the promoters of genes, such as vinculin and SRF itself, that are activated by Rho–actin signalling.

That actin and Rho–actin-induced nuclear translocation of MAL required the RPEL motifs of MAL implied that MAL and actin might interact. They did, through MAL's RPEL motifs and independently of actin's polymerization ability. Treisman and colleagues then showed that upstream Rho–actin signalling dissociated the actin–MAL complex. So, a simple model for MAL localization by Rho–actin signalling can be envisaged: the level of unpolymerized G-actin itself could regulate MAL localization and prevent it translocating to the nucleus — depletion of the pool of G-actin, by Rho-induced polymerization, would cause MAL nuclear accumulation. It remains possible that, in addition, a distinct positively acting G-actin subpopulation could also promote MAL nuclear import. The authors speculate that MAL signalling controls a subset of SRF targets that influence cell morphology, adhesion and movement. As MAL is rearranged in t(1;22)(p13;q13) leukaemia, dysregulated expression of such targets might well form the basis for transformation in this condition.

Katrin Bussell

References

Miralles, F. et al. Actin dynamics control SRF activity by regulation of its coactivator MAL. Cell 113, 329–342 (2003) | PubMed |