Providing for protein synthesis Compartmentalization of metabolic processes into organelles can have important consequences. Zhu et al. examined the role of the coenzyme nicotinamide adenine dinucleotide phosphate (NADP + ) and its reduced form (NADPH) in cultured human cells. They found that cells lacking NAD kinase 2, an enzyme needed to make NADPH, had decreased abundance of mitochondrial NADPH and proliferated slowly in culture medium with limited nutrients because of a lack of proline. Proline is made in the mitochondria, and thus a key function of NADPH in the mitochondria appears to be the synthesis of proline to sustain cellular protein synthesis. Science , abd5491, this issue p. 968