Optical tweezers unveil a secret of muscle power
Our hearts beat a life long. With every beat our heart muscle contracts and expands. How this can work throughout an entire life remains largely a mystery. Researchers at the Technical University of Munich (TUM) have now measured the forces acting between the building blocks titin and α-actinin which stabilize the muscle.
The human body is a never-ending construction site: Proteins are permanently being decomposed and replaced. But this perpetual reconstruction does not inhibit the body's functionality. The heart keeps beating, respiration does not halt, we can rely on our eyes and ears around the clock.
How the body manages to keep together the protein strands in muscles even while individual building blocks are replaced has preoccupied Prof. Matthias Rief for many years. "There must be forces that stabilize the individual chains, the filaments. Otherwise, muscles would fall apart. But, until now, nobody has tracked down the source of these forces," reports the chair of the Department of Biophysics at TU Munich. In collaboration with his team, he has now deciphered the secret behind the cohesion of muscles.
Optical tweezers decipher the bonding forces
As it turns out, two proteins are responsible for allowing muscles to expand without falling apart. The first is titin, the longest protein in the human body. The second is α-actinin, which anchors titin to the muscle tissue.
The TUM researchers studied the interactions between these proteins using a specially developed apparatus: The "optical tweezers" fill a 20-square meter room in the basement of the institute. There are laser sources, optics, cameras and monitors. The heart of the facility is a measuring chamber filled with a fluid and small globules of glass. The titin and α-actinin molecules stick to the surface of these globules. Pairs of laser beams that penetrate the measuring cell catch one globule each to hold them fast.
Marco Grison et al, α-Actinin/titin interaction: A dynamic and mechanically stable cluster of bonds in the muscle Z-disk, Proceedings of the National Academy of Sciences (2017). DOI: 10.1073/pnas.1612681114
Two laser beams hold tiny glass beads in a position very close to each other. One end of the protein complex is fixed on the surface of the left brad, the other end on the right. If the laser moves the glass beads apart, the protein molecules are forced to stretch and the forces can be measured. Credit: Marco Grison / TUM