The prohibitins, called PHB1 and PHB2, are highly conserved proteins that are present in multiple compartments in eukaryotic cells. PHB1 is 30kDa tumor suppressor protein involved in cell cycle control. PHB1 has been found in mitochondria, the nucleus and the plasma membrane, as well as extracellularly in circulation. In mitochondria prohibitins mainly exist as membrane-bound ring complexes and function as chaperones maintaining mitochondrial protein stability during protein synthesis and transportation. In the nucleus prohibitins interact with transcription factors such as Rb and p53 to regulate target gene transcription. Extracellular prohibitin can bind and activate C3 to enhance complement activation.