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Original Article

Nada Jabado^a, Andrzej Jankowski^b, Samuel Dougaparsad^b, Virginie Picard^a, Sergio Grinstein^b, and Philippe Gros^a

^a Department of Biochemistry, McGill University, Montreal H3G-1Y6, Quebec, Canada
^b Division of Cellular Biology, The Hospital for Sick Children, Toronto M5G 1X8, Ontario, Canada
Department of Biochemistry, McGill University, 3655 Drummond St., Montreal H3G-1Y6, Quebec, Canada.514-398-2603514-398-7291

Mutations at the natural resistance–associated macrophage protein 1 (Nramp1) locus cause susceptibility to infection with antigenically unrelated intracellular pathogens. Nramp1 codes for an integral membrane protein expressed in the lysosomal compartment of macrophages, and is recruited to the membrane of phagosomes soon after the completion of phagocytosis. To define whether Nramp1 functions as a transporter at the phagosomal membrane, a divalent cation-sensitive fluorescent probe was designed and covalently attached to a porous particle. The resulting conjugate, zymosan–FF6, was ingested by macrophages and its fluorescence emission was recorded in situ after phagocytosis, using digital imaging. Quenching of the probe by Mn²⁺ was used to monitor the flux of divalent cations across the phagosomal membrane in peritoneal macrophages obtained from Nramp1-expressing (+/+) and Nramp1-deficient (–/–) macrophages. Phagosomes from Nramp1^+/+ mice extrude Mn²⁺ faster than their Nramp^–/– counterparts. The difference in the rate of transport is eliminated when acidification of the phagosomal lumen is dissipated, suggesting that divalent metal transport through Nramp1 is H⁺ dependent. These studies suggest that Nramp1 contributes to defense against infection by extrusion of divalent cations from the phagosomal space. Such cations are likely essential for microbial function and their removal from the phagosomal microenvironment impairs pathogenesis, resulting in enhanced bacteriostasis or bactericidal activity.

Key Words: Nramp1 • transporter • divalent cation • phagosome • V-ATPase

© 2000 The Rockefeller University Press

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