Cells use photosynthesis to produce energy-rich sugar from energy-poor inorganic molecules with the help of sunlight. This process occurs in the photosynthetic organelles of plants and algae, i.e. the plastids. These evolved over 1.5 billion years ago when a host cell assimilated a cyanobacterium capable of photosynthesis. It was in a similar way that the mitochondria, the cell’s powerhouses, also formed around two billion years ago.
What happened to the cyanobacterium 1.5 billion years ago? Evidently the assimilating cell did not digest the prey bacterium but instead kept it alive and combined symbiotically with it. This ultimately continued to such a degree that the bacterium lost a major part of its own genome and transferred parts of it into the cell nucleus of the host cell. It was then the host cell’s task to read this genetic information and synthesise proteins from it that are imported into the plastids and essential for their structure and function.
The team of biologists in Düsseldorf led by Dr. Eva Nowack, together with colleagues in Marburg, reconstructed the evolutive processes which led to the integration of the plastids in the host cell. What was difficult here was the fact that this took place a very long time ago. However, the amoeba Paulinella chromatophora came to their assistance: Its photosynthetic organelles – what are referred to as chromatophores – only developed around 100 million years ago. The chromatophores display properties which characterise them as an intermediate state between cyanobacteria and plastids.
In an article in Current Biology, the researchers show that the chromatophores in Paulinella chromatophora, despite their recent (on an evolutionary scale) integration, already imported hundreds of proteins that are encoded in the cell nucleus of the amoeba and synthesised by it. In the case of long imported proteins (with over 200 amino acid building blocks) – of which many perform metabolic functions – the researchers discovered a special and important feature. They have a signal sequence which presumably marks them for import into the chromatophore. This sequence is the key, so to speak, with which the proteins can get past the envelope separating the chromatophore from the rest of the cell’s interior.
The chromatophoric import signal differs greatly from the signals which in plant cells control the import of proteins into the plastids there. It is nevertheless possible to channel proteins into plastids too by means of the chromatophoric import signal. To this purpose, the researchers attached the chromatophoric import signal to special fluorescent proteins and observed that the proteins prepared in that way were assimilated by plastids in tobacco plants. This suggests that the protein import mechanism in chromatophores and plastids makes use of common remaining elements from cyanobacteria and the host cells.
Dr. Nowack, head of the Emmy Noether Junior Research Group “Microbial Symbiosis and Organelle Evolution” at HHU, points out a further significant result: “Hardly any proteins imported into the chromatophore stem from its cyanobacterial precursors. It’s far more the case that most of them come from the host cell, but partly also from other ‘prey bacteria’. What this means it that proteins from different sources were mixed as the chromatophore developed. That’s another reason why it can be difficult to decode the evolutionary origins of various cell organelles.”
Original publication
A. Singer, G. Poschmann, C. Mühlich, C. Valadez-Cano, S. Hänsch, V. Hüren, S. A. Rensing, K. Stühler & E. C. M. Nowack, Massive protein import into the early-evolutionary-stage photosynthetic organelle of the amoeba Paulinella chromatophora, Current Biology 27, 1–11, 25 September, 2017.