Role for heme in mammalian protein synthesis: activation of an initiation factor

Two nonerythropoietic tissues, brain and liver, contain an initiation factor that can overcome the block in initiation of protein synthesis seen in reticulocyte lysates when exogenous hemin is not present. Upon incubation of the brain factor with hemin and removal of free hemin by gel filtration, the factor activity is strongly stimulated. This stimulation shows a concentration dependence on hemin close to that seen for stimulation of protein synthesis in whole reticulocyte lysates. The data indicate that hemin mediates the formation of an active initiation factor complex from inactive, lower mol wt components.