Abstract: The invention relates to methods for identifying nucleic acid molecules that encode mitochondrial divalent cation transporter polypeptides, which in preferred embodiments are mitochondrial calcium uniporters. Preferred methods employ host cells for expression screening of nucleic acid libraries encoding candidate cation transporters, where the host cells comprise a mitochondrially targeted divalent cation-sensitive indicator molecule. In certain preferred embodiments host cells comprise a mitochondrially targeted esterase that is capable of cleaving a divalent cation sensitive indicator molecule precursor to provide a divalent cation sensitive indicator molecule, or an expression construct directing the expression of such esterase. Host cells that lack endogenous calcium uniporter activity, or that exhibit impaired cell growth in the presence of cytostatic Ca2+ concentrations, are also described.