Among the most studied protein machines in history, mTORC1 has long been known to sense whether a cell has enough energy to build the proteins it needs to multiply as part of growth. Because faulty versions of mTORC1 contribute to the abnormal growth seen in cancer, drugs targeting the complex have been the subject of 1,300 clinical trials since 1970. Now, a new study finds that mTORC1 has a second function of profound importance: controlling how "crowded" human cells become. The finding explains for the first time the workings of a physical quality that cells use to regulate their actions, and more closely links malfunctions in mTORC1-related genes to several diseases of aging. Based on past studies, biologists have long concluded that cells, for survival, require a limit on the number of proteins in their fluid-filled inner spaces, the cytoplasm, where many cellular functions occur. Specifically, the current study found that the mTORC1 complex controls crowding by determining the number of ribosomes, multiprotein machines that build other proteins there.
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