Researchers discovered that a protein called leiomodin (Lmod) promotes the assembly of a heart muscle protein called actin. Lmod was also found to direct the assembly of actin to form the pumping unit of the heart.
Actin is the most abundant protein in most animal cells and forms long polymers that make up the cell skeleton. It requires nucleator proteins to control the time and place where actin filaments forms. In cells that make up muscles and the heart, interactions of actin filaments with motor proteins produce contractions that pump blood through the body.
The team compared the amino acid sequence of Lmod with that of another protein called tropomodulin (Tmod) that was already known to bind actin filaments in muscle cells. They found that one part of Lmod was very similar to Tmod but that Lmod was a bigger protein. They also observed that Lmod contained unique features, making the scientists suspect that it could assemble the actin filaments of the heart muscle. Further research showed that Lmod can indeed serve as the nucleator protein to initiate the forming of actin polymers in heart muscle cells.
The investigators also found that Lmod directs actin filaments to the sarcomere, the part of the heart that controls contractions or pumping. When Lmod was knocked down in cardiac muscle cells, the sarcomeres became completely disorganized and could not direct muscles to contract.
Researchers involved in this study are from the University of Pennsylvania School of Medicine, Boston Biomedical Research Institute, University of Helsinki, and Yale University. The findings appear in the April 11 issue of Science.