The cyclic nucleotide–binding homology domain of the integral membrane protein CNNM mediates dimerization and is required for Mg2+ efflux activity — Structure of CNNM cyclic nucleotide–binding homology domain — The cyclic nucleotide–binding homology domain of the integral membrane protein CNNM mediates dimerization and is required for Mg2+ efflux activity — Structure of CNNM cyclic nucleotide–binding homology domain