Two of the crucial components for the origin of life – genetic material and cell membranes – could have been introduced to one another by a lump of clay, new experiments have shown.
The study of montmorillonite clay, by Martin Hanczyc, Shelly Fujikawa and Jack Szostak at the Massachusetts General Hospital in Boston, revealed it can sharply accelerate the formation of membranous fluid-filled sacs.
These vesicles also grow and undergo a simple form of division, giving them the properties of primitive cells. Previous work has shown that the same simple mineral can help assemble the genetic material RNA from simpler chemicals. “Interestingly, the clay also gets internalised in the vesicles,” says Leslie Orgel, an origin of life expert at the Salk Institute for Biological Sciences in San Diego, California. “So this work is quite nice in that it finds a connection between the mechanism that creates RNA and encloses it in a membrane.”
Inherit, mutate, evolve
The genesis of genetic material and the emergence of cell structure are hot areas of research, but until now the two had not connected. The birth of genetic material was clearly crucial for life to take on its unique abilities to inherit, mutate and evolve.
And membranes were key to the physiology of cells because they protect their contents, concentrate chemicals to promote reactions and isolate successful genes from unsuccessful ones. “It’s clear you really need both these elements to get evolution off the ground and running,” says Szostak.
Research has already shown that some of building blocks for RNA-like molecules and membranes are spontaneously created by chemical reactions in outer space and in conditions that may have existed on the primordial Earth. But how these subunits were then assembled is still debated.
For RNA, one popular theory revolves around the unusual properties of montmorillonite clay. The negatively charged layers of its crystals create a sandwich of positive charge between them. This turns out to be a highly attractive environment for RNA subunits to concentrate and join together into long chains.
Szostak wondered whether montmorillonite could also help the assembly of vesicles from simple fatty acid precursors. He remembers the day his colleagues Hanczyc and Fujikawa ran into his office to show him their first results: the clay caused a 100-fold acceleration of vesicle formation.
“It was pretty amazing,” he says. Once formed, the vesicles often incorporated bit of clay and were able to grow by absorbing more fatty acid subunits.
His team also showed the clay could hold RNA and form vesicles at the same time. Fluorescently-labelled RNA attached to the clay ended up assembled into vesicles after the reaction. And the researchers were able to get these “protocells” to divide by forcing them through small holes. This caused them to split into smaller vesicles, with minimal loss of their contents.
Szostak admits that in a natural setting the vesicles would rarely be forced to divide in this way. So now his group is searching for different mixtures of membrane-forming molecules that might divide spontaneously when they reach a certain size.
Journal reference: Science (vol 302, p 618 )