Abstract:

The sarcomere is the minimal contractile unit in the cardiac and skeletal muscle, where actin and myosin filaments slide past each other to generate tension. This molecular machinery is supported by a subset of highly organised cytoskeletal proteins that fulfil architectural, mechanical and signalling functions. The ultra-structure of sarcomere is highly organized and delimited by Z-disks, which play a central role in the mechanical stabilization and force transmission.

In the Z-disks – the lateral boundaries of the sarcomere machinery – the protein α-actinin-2 cross-links antiparallel actin filaments from adjacent sarcomeres, and additionally serves as a binding platform for a number of other Z-disk proteins. In striated muscle cells, the Z-disk represents a highly organized three-dimensional assembly containing a large directory of proteins orchestrated in a multi-protein complex centred on its major component α-actinin, with still poorly understood hierarchy and three-dimensional interaction map. On the way to elucidate the molecular structural architecture of the Z-disk, the hierarchy of its assembly and structure-function relationships, we are studying binary and higher order sub-complexes of α-actinin using a combination of molecular biophysics, structural and biochemical approaches.

FATZ proteins interact with α-actinin and five other core Z-disk proteins, contributing to myofibril assembly and maintenance as a protein interaction hub. We determined the first structure and its cellular validation of α-actinin-2 in complex with a Z-disk partner, FATZ-1, which is best described as a conformational ensemble. We show that FATZ-1 forms a tight fuzzy complex with α-actinin-2 and propose a molecular interaction mechanism via main molecular recognition elements and secondary binding sites. The obtained integrative model reveals a polar architecture of the complex which, in combination with FATZ-1 multivalent scaffold function, might organise interaction partners and stabilise α-actinin-2 preferential orientation in the Z-disk. Finally, we uncover FATZ-1 ability to phase-separate and form biomolecular condensates with α-actinin-2, raising the intriguing question whether FATZ proteins can create an interaction hub for Z-disk proteins through membrane-less compartmentalization during myofibrillogenesis.

I will present our studies on the interaction of the major Z-disk protein α-actinin with FATZ and Z-portion of titin, forming dynamic fuzzy complexes, and discuss findings in view of asymmetric sorting of α-actinin and sarcomeric Z-disk architecture and assembly.

Bio:

Kristina Djinović Carugo received her PhD in Structural Biochemistry from the University of Ljubljana, Slovenia in 1992, with research performed at the University of Pavia, Italy. After the post-doctoral stage at the University of Pavia, she moved to EMBL in Heidelberg, Germany (1995), as EMBO post-doctoral fellow and then staff scientist. In 1999 she joined Elettra - Sincrotrone Trieste, Italy, to head the Unit of Structural Biology and Crystallography. Since 2004 she has been holding the Chair of Molecular Structural Biology at the University of Vienna and was 2009 – 2022 the Head of the Department of Structural and Computation. Since 2022 she is the Head of EMBL Grenoble, while remaining 20% engaged with University of Vienna.

Her main scientific interests are the molecular mechanisms underlying the architecture and assembly of the Z-discs -the boundaries between the basic contractile units of striated muscle - the sarcomeres. She combines biochemical, molecular biophysics and structural biology methods on reconstituted complexes and their components in integrative structural modelling of molecular assembles. Other long standing research lines are structure-function analysis of metallo-enzymes involved in protection from chemical and oxidative damage and X-ray-induced radiation damage mechanisms of metallo-centres. At Elettra she also investigated the use of soft X-rays in macromolecular crystallography.