A, Transport systems of magnesium (Mg). Specific membrane-associated Mg transport proteins only have been described in bacteria such as Salmonella. Although similar transport proteins are believed to be present in mammalian cells based on nucleotide sequence analysis, they have not yet been demonstrated. Both MgtA and MgtB (molecular weight, 91 and 101 kDa, respectively) are members of the adenosine triphosphatase (ATPase) family of transport proteins. B, Both of these transport proteins have six C-terminal and four N-terminal membrane-spanning segments, with both the N- and C-terminals within the cytoplasm. Both proteins transport Mg with its electrochemical gradient, in contrast to other known ATPase proteins that usually transport ions


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