Knowing where proteins are localized in the cell can help map out cellular processes and signaling pathways to better understand the flow of information within and between cells. The subcellular location of a protein is thought to be tightly linked to its function so that a protein in the nucleus may be involved in gene regulation, while one in the mitochondria might play a role in energy production. However, after having studied the subcellular location of the majority of all human proteins it seems that more than half are in fact multilocalizing proteins (MLPs).

MLPs are found in multiple subcellular locations and is an interesting and rather diverse group of proteins that includes e.g. shuttling proteins that transport other proteins between organelles, proteins involved in processes or reactions shared between different organelles, and also the intriguing "moonlightning" proteins that not only are multilocalizing but also have different functions in the different locations.

The fraction of MLPs is not equal between organelles but might to a certain extent reflect how they function. A more self-contained compartment like mitochondria seem to have a smaller fraction of MLPs than e.g. plasma membrane, cytosol and nucleus that rely on proteins acting across organelle borders to e.g. regulate metabolic reactions and control gene expression.

Read more about the multilocalizing proteins, in which organelles they reside and how they function here.