Proteins and the hidden inner worlds they create

What are protein condensates?

Biomolecular condensates are small ‘compartments’ created inside cells without any membrane  playing key roles to organise myriad biochemical reactions. Unlike typical cell structures, these condensates are formed due to a process called liquid-liquid phase separation, where specific proteins clump together (like oil droplets in water). This phenomenon is studied worldwide but how exactly the sequence of amino acids in these proteins defines the condensates’ intricate structure, stability, and their internal environment is a large mystery.

This study aims to bridge that gap by scientifically investigating how simple, engineered protein models named diblock elastin-like polypeptides (ELPs) organise themselves into these clusters and what it reveals about the core principles dictating condensate structure.

Research Question and Purpose

The primary research question driving the study was:How does the chemical composition of protein sequences influence the internal structure and stability of biomolecular condensates?

The function, structure and essentially all defining characteristics of any protein is dependent on its amino acid sequence. Therefore, understanding the relationship between sequence and condensate behaviour is crucial for any study and many cellular processes require extremely precise control over condensate formation. Despite this, existing theories such as Flory-Higgins and models such as ‘sticker-spacer’ do not explain the complete picture of this phenomenon; specifically, they do not explain the micro-scale organisation inside condensates.

The Method

The study approaches the RQ by combining 2 methods:

1. Multiscale simulations — scientists make use of computer models that explore how different sequences of ELPs cluster together and interact with water at varying degrees of detail.

2. Fluorescence lifetime imaging microscopy (FLIM) — this involves lab experiments where molecules tagged with dyes that are sensitive to environmental changes reveal and highlight distinct internal environments of the condensates. 

By engineering the ELPs with varying amino acid sequences and observing the outcomes, the authors track the exact relationship between the 2 variables in their RQ.

Key Findings

After studying the results of the experiments, scientists could draw these conclusions:

• Condensate stability varied in a predictable manner when certain amino acids were substituted in the polypeptide chain and these variations matched simulation predictions (Simulations accurately modelled experimental stability pattern).

• Condensates are not uniform throughout; they contain distinct microregions displaying varying chemical characteristics (Heterogenous internal environments).

• The phase boundary did not distinctively separate the 2 liquids as expected. Instead of forming separate hydrophobic and hydrophilic phases, the protein and surrounding water formed a mixed interfacial zone with many water molecules still attached to the protein chain via hydrogen bonding (even in regions that were expected to be hydrophobic (Interfacial environment visibly dominate physical separation).

• There was observed to be a strong correlation between condensate stability and a hydrophobicity scale that reflects some interfacial transfer free energies. This simply implies that interfacial properties matter more than just simple phase segregation.

Limitations

• Since the study makes use of engineered ELPs and not natural condensates, the results may not account for all the complexities occurring in cellular condensates.

• When using multiscale modeling, simulations work under certain assumptions about interactions and simplify molecular details for convenience of study (there is a higher degree of control over variables). However, these may differ in living cells.

• Similarly, experiments were done in vitro and the experimental environment may not mirror exact cellular environments which have more interacting molecules and chemical regulation.

These limitations only show that the study helps establish a foundational explanation behind the phenomenon instead of offering complete descriptions of natural biomolecule condensates.

Implications of the study

• Better understanding of formation mechanisms can illuminate how exactly condensates play a larger role in several other processes such as gene expression, stress control, and signal transduction.

• The innovative approach of combining simulations with microscopy provides a template for studying other biomolecular assemblies.

• Insights into interfacial environments can help design new biomaterials or methodologies that depend on controlled phase separation (ex. Targeted drug delivery).

Essentially, this work brings us closer to decoding how the protein ‘code’ determines where and how condensates form and function which is a significant question with relevance across cell biology and biotechnology.

Adi Vinay Chousalkar | Writer | The STEM Review