Transcriptional activity of megakaryoblastic leukemia 1 (MKL1) is repressed by SUMO modification

Genes Cells. 2005 Aug;10(8):835-50. doi: 10.1111/j.1365-2443.2005.00880.x.


Megakaryoblastic leukemia 1 (MKL1) was originally identified as a gene translocated in megakaryoblastic leukemia. It has been shown that MKL1 functions as a RhoA-regulated transcriptional coactivator of serum response factor (SRF). In order to identify a protein that regulates the function of MKL1, we performed yeast two-hybrid screening and isolated cDNA that encodes UBC9, an E2 enzyme of small ubiquitin-related modifier-1 (SUMO-1), as an MKL1-binding protein. UBC9 was found to physically interact with MKL1 by GST pull-down assay, and MKL1 was covalently modified with SUMO-1 in 293T cells and in vitro reconstitution system. MKL1 sumoylation is enhanced by either serum stimulation or co-expression of constitutively active form of RhoA. Mutational analysis showed that lysine residues at 499, 576, and 624 are the major acceptor sites for SUMO-1. In addition, reporter gene analysis revealed that mutation of the three sumoylation sites strongly enhances the transcriptional activity of MKL1. The covalent attachment of SUMO-1 to MKL1 by gene fusion represses MKL1-dependent transcription in a complementary manner. Finally, mutation of the sumoylation sites of MKL1 also enhances SRF-dependent transcription without affecting MKL1-SRF interaction. The combined results demonstrated that MKL1 is sumoylated and this modification represses transcriptional activity of MKL1.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cells, Cultured
  • Cycloheximide / pharmacology
  • DNA-Binding Proteins / genetics
  • DNA-Binding Proteins / metabolism*
  • Down-Regulation
  • Genes, Reporter
  • HeLa Cells
  • Humans
  • Leukemia, Megakaryoblastic, Acute / metabolism*
  • Oncogene Proteins, Fusion / genetics
  • Oncogene Proteins, Fusion / metabolism
  • SUMO-1 Protein / metabolism*
  • Serum Response Factor / metabolism
  • Time Factors
  • Transcription, Genetic*
  • Transfection
  • Tumor Cells, Cultured
  • Two-Hybrid System Techniques
  • Ubiquitin-Conjugating Enzymes / metabolism
  • rhoA GTP-Binding Protein / metabolism


  • DNA-Binding Proteins
  • Oncogene Proteins, Fusion
  • SUMO-1 Protein
  • Serum Response Factor
  • Cycloheximide
  • Ubiquitin-Conjugating Enzymes
  • rhoA GTP-Binding Protein
  • ubiquitin-conjugating enzyme UBC9