Transcriptional activation of SRE-dependent promoters by MKL1 and binding of SRF to MKL1. (A) SRE-dependent or -independent luciferase reporter genes (0.5 μg), together with pRL-SV40P (50 ng) as an internal control, were transiently transfected into HeLa cells with vector (pcDNA3) or an MKL1 expression plasmid, p3×Flag-MKL1 (1 μg). Two days after transfection, luciferase assays were performed and normalized to the Renilla luciferase control. The values are expressed as the increase with MKL1 compared to the vector. Data are represented as the mean ± standard deviation of three independent experiments. α-SA, smooth muscle α-actin; α-CA, cardiac α-actin; ANF, atrial natriuretic factor. (B) Binding of MKL1 to SRF in gel mobility shift assays. Gel mobility shift assays were performed with 32P-labeled oligonucleotide probe for a high-affinity SRF binding site (XGL) with bacterially expressed SRF(114-508) and/or in vitro translation products of MKL1 (5 and 8 μl in lanes 2 and 3, respectively) or control translation lysates (5 μl) as indicated. (C) Coimmunoprecipitation of MKL1 and SRF. HA-tagged SRF and 3×Flag-tagged MKL1 (2 μg each) were transfected into HeLa cells alone (with 2 μg of control plasmid pEGFP-N1) or together as indicated. Immunoprecipitates (IP) with anti-Flag antibodies were immunoblotted (IB) for SRF with anti-HA antibodies (top) and reprobed with anti-Flag antibodies to determine the presence of Flag-tagged MKL1 (bottom). One-thirtieth of the cell lysates was directly immunoblotted with anti-HA antibodies to detect HA-tagged SRF (middle). The positions of HA-SRF, Flag-MKL1, and the immunoglobulin heavy chain (Ig) are indicated. (D) Coimmunoprecipitation of endogenous SRF and MKL1. HeLa cell lysates were immunoprecipitated with nonspecific rabbit serum (NS) or rabbit antiserum to SRF or MKL1 and immunoblotted with anti-MKL1 serum. For lane 3, 1/10 as much lysate was used. MKL1 migrated at 160 kDa relative to markers.