Is a membraneless chloroplast organelle

Molecular grammar of phase separation

Research Report 2018 - Max Planck Institute for Molecular Cell Biology and Genetics

Hyman, Anthony; Alberti, Simon
Organization of cytoplasm
A cell needs spatially separate reaction areas so that different reactions can take place undisturbed from one another. This is also possible without a biomembrane: proteins and RNAs can be enriched in condensates by phase separation and fulfill their various functions. The mechanisms of this phase separation hold the key to some of the biggest open questions in biology and pave the way for revolutionary developments in cell physics. We have deciphered a molecular grammar that underlies the phase separation of some proteins.

introduction

If you look at a village full of people, they are doing their different jobs in different places. In order to function, a cell also has to carry out different metabolic activities at the same time. In order for these reactions to proceed undisturbed from one another, the cell is known to have various organelles, such as mitochondria and chloroplasts, which are enclosed by a lipid membrane. There is, however, another possibility: compartments without membranes, which are created by the segregation (phase separation) of certain liquid cell components and thus behave similarly to oil droplets in water.

Our research group led by Anthony Hyman is researching which proteins control the formation of such liquid compartments. In addition, the team around Simon Alberti, a former Max Planck researcher and now at the Biotechnological Center of the TU Dresden, is mainly interested in proteins that do exactly the opposite and clump together. That they do this is because they contain sections that cause the proteins to fold incorrectly and solidify into less dynamic gels over time - a mechanism previously known from prions. This category includes the protein FUS, which is also associated with ALS and other age-related neurodegenerative diseases. In healthy cells, FUS behaves like a liquid: it forms spherical drops, the molecules in it can move freely, and if two drops come close, they merge into one large drop. However, in ALS patients, FUS changes its physical state and becomes solid.

So far, it has not been fully understood which sequence-specific code of amino acids or which amino acids drive a phase separation or trigger a solidification process and thus control the material properties of biomolecular compartments. It has therefore been difficult to influence the phase separation properties in cells up to now. This in turn made it more difficult to further research the biological functions of the compartments.

The molecular grammar is in the protein sequence

Together with the research group of the Biotechnological Center of the TU Dresden and Jeong-Mo Choi and Alex Holehouse from the Rohit Pappu working group at Washington University in St. Louis, USA, we used a combination of experimental and theoretical analyzes to develop a molecular grammar of the FUS- Proteins discovered. This molecular grammar is contained in the protein sequence and ensures that liquid components separate through phase separation during the formation of organelles. We found out that the amino acid residues tyrosine and arginine in particular regulate the phase separation. Furthermore, the amino acid glycine ensures that protein droplets remain liquid, while the amino acids glutamine and serine promote solidification. Jie Wang, first author of the study, explains: "So we found that the phase separation behavior of prion-like RNA-binding proteins is determined by certain amino acids. Furthermore, the theoretical work of the study shows that the phase behavior was predicted by the chemistry of these amino acids can be." In addition, our team used targeted mutations to develop polymer models that influence the phase separation of proteins in cells. It was relatively easy to change the driving forces for phase separation and the material properties of FUS proteins through this mutation.

The amino acid sequence can predict phase separation

The results of the study suggest that in the near future, scientists will be able to predict and study the properties of phase separation based on amino acid sequences. This could soon make it possible to identify and develop proteins with pronounced phase separation properties solely on the basis of their protein sequences. These proteins can then be introduced into living cells to study the function and susceptibility of biomolecular condensates to disease. This would enable insights into the mechanisms of age-related diseases in the future.

Bibliography

Wang, J .; Choi, J.M .; Holehouse, A.S .; Zhang, X .; Jahnel, M .; Lemaitre, R .; Maharana, S .; Pozniakovsky, A .; Drechsel, D .; Poser, I .; Pappu, R.V .; Alberti, S .; Hyman, A.A.
A molecular grammar underlying the driving forces for phase separation of prion-like RNA binding proteins