Prebiotic peptide mimics as building blocks for membraneless droplets

Abstract

The emergence of compartmentalization is considered as a crucial step in the transition from prebiotic chemistry to early cellular life. One of the widely proposed models for protocellular compartment is membraneless droplets. These are also known as coacervates, which are formed through liquid–liquid phase separation and can concentrate prebiotically relevant molecules, and facilitate biochemical reactions. In this study, we constructed a peptide-based complex coacervate model system, using a short peptide sequence, DFDGDAD that is inspired from prebiotically plausible peptides. This peptide contains the ancestral DGD motif which is universally conserved in the active sites of RNA polymerases of all living cellular organisms and also in some modern enzymes. The primary objective of this study was to investigate whether such a short peptide could form coacervates in isolation or with another molecule like polylysine, allowing for the resultant compartments to function as a minimal protocellular model capable of molecular sequestration. Primordial life likely emerged in chemically heterogeneous environments where peptides, nucleotides and other small metabolites coexisted. Therefore, to better mimic this complexity, ATP was introduced as a co-solute in the coacervates that resulted from the aforementioned. The stability of the resultant coacervates from both the approaches was evaluated against prebiotically plausible selection pressures, including high salt concentrations and exposure to a range of pHs. Further, to assess droplet stability and functionality, their ability to sequester cargo molecules with different physicochemical properties was evaluated. Both the systems tolerated moderate salt concentrations and sequester RNA and hydrophilic dyes efficiently, underscoring their relevance as membraneless compartments that could have existed and functioned under early Earth conditions. Such model systems could also allow for systematic investigation of reaction kinetics and enzymatic activity in biologically relevant molecules, shedding light on the capacity of short sequences to result in functional microreactors.