Estimating the number of protein molecules in a plant cell: a quantitative perspective on proteostasis and amino acid homeostasis during progressive drought stress

During dehydration cellular proteostasis as well as amino acid homeostasis are severely challenged, since the decrease in photosynthesis induces massive proteolysis. Thus, we selected progressive drought stress in Arabidopsis thaliana as a model to investigate the balance between protein and free amino acid homeostasis on a quantitative level. We analyze the mass protein composition of rosette leaves and estimate, how many protein molecules are present in a plant cell and its subcellular compartments. Under control conditions, an average Arabidopsis mesophyll cell contains about 25 billion protein molecules and 80% of them are localized in the chloroplasts. Severe water deficiency leads to degradation of more than 40% of the leaf proteome and thus causes a drastic shift towards the free amino acid pool. Stress induced proteolysis of half of the 400 million RubisCO hexadecamers present in the chloroplasts of an individual mesophyll cell alone doubles the cellular content in free amino acids. A major fraction of the amino acids released from proteins is channeled into the synthesis of proline as a compatible osmolyte. Complete oxidation of the remaining part as an alternative respiratory substrate can fully compensate the lack of carbohydrates derived from photosynthesis for several hours.