Macromolecular complexes are naturally occurring machines inside cells. They consist of a handful to several thousand individual components, including proteins, DNA, carbohydrates and lipids, and perform diverse and vital tasks, such as translating the genetic code, converting energy or helping nerve cells communicate.
Some well-known complexes help regulate gene expression via effects on RNA and proteins. The spliceosome (Fig. 1; ref 1), for example, removes non-protein-coding snippets from newly formed RNA, then joins the remaining fragments to form functional messenger RNA (mRNA) that can be converted into protein.
The nuclear-pore complex — one of the largest molecular machines — straddles the nuclear membrane, controlling the exit of RNA and the entrance of other molecules including proteins and signalling molecules (Fig. 2; ref 2).
The ribosome (Fig. 1; refs 3,4) binds to and moves along the mRNA template, reading its genetic information and preparing the corresponding amino-acid sequence, which it then stitches together to make protein. Unwanted RNA is broken down by another macromolecular complex, the exosome (Fig. 1; ref 5), and unwanted proteins are recycled by the proteasome.
An interesting group of macromolecular complexes exist within the lipid bilayer membrane that surrounds the cell and its internal compartments. Photosynthetic membrane complexes can be found in plant chloroplasts and bacterial membranes. They convert solar energy into chemical energy6, which can be used to make organic compounds — the building blocks of life.
Another group, found in the plasma membrane of bacteria and the mitochondrial membrane of eukaryotic cells, extracts energy from cellular respiration. During this process, electrons are transferred from organic substrates to molecular oxygen, generating a proton gradient across the membrane, which in turn helps the ATP synthase macromolecular complex to produce chemical energy, providing animal cells with the energy to live.