A key question in biology is how the billions of molecules floating around inside a cell are coordinated in space and time to carry out the myriad biochemical reactions required for proper cellular function. Intracellular organization has for decades been associated with compartments surrounded by lipid membranes that provide unique environments in which molecules can be sequestered to perform specific processes. As scientists have discovered in recent years, a membrane barrier is not the only mechanism by which cells can partition their components. Driven by a phenomenon called liquid-liquid phase separation, biomolecular condensates form membrane-less compartments that are increasingly implicated in many biological processes and human diseases.

"Although liquid-liquid phase separation has been observed in a number of biological systems, it has not been reported in striated muscles until recently," says Kristina Djinovic-Carugo. "We have discovered that sarcomeric Z-disc assembly in vitro involves phase separation, raising the tantalizing question of whether sarcomere biogenesis begins with assemblies in macromolecular condensates." The sarcomere is the basic contractile unit of striated muscle and is anchored in Z-discs, which are highly ordered, multiprotein structures that form the boundaries of adjacent sarcomeres. Z-discs have been observed to form from Z-bodies, tiny puncta reminiscent of biomolecular condensates, that contain actin and FATZ-1, an intrinsically disordered protein. The team will now investigate how a highly ordered structure like the Z-disc could arise from disordered assemblies.

Kristina Djinovic’s group will use their established protocols for the in vitro generation of sarcomeric proteins and combine them with the lab’s expertise in structural biology to address the question of sarcomere assembly. Co-applicants John Hinson (University of Connecticut, USA) and Toshiyuki Oda (University of Yamanashi, Japan) bring complementary expertise in human genetics, tissue engineering, computational methods, and cryo-ET to the project. Finally, team member Jonas Ries, who will establish his lab at the Max Perutz Labs later this year, is an expert in super-resolution microscopy. "Our microscopy technologies will allow us to see where specific proteins are located in the Z-disc and how this organization changes dynamically over time. I am very excited to develop our methods for this specific biological application, to be able to observe what no current method can," he concludes.

Human Frontier Science Program

Kristina Djinovic Lab

Jonas Ries Lab