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Molecular and Cellular Biology, November 2000, p. 7893-7902, Vol. 20, No. 21
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Saccharomyces cerevisiae Expresses Three Functionally Distinct Homologues of the Nramp Family of Metal Transporters

Matthew E. Portnoy,1 Xiu Fen Liu,2,dagger and Valeria Cizewski Culotta1,2,*

Departments of Biochemistry and Molecular Biology1 and Environmental Health Sciences,2 Johns Hopkins University School of Public Health, Baltimore, Maryland 21205

Received 7 June 2000/Returned for modification 13 July 2000/Accepted 31 July 2000

The baker's yeast Saccharomyces cerevisiae expresses three homologues of the Nramp family of metal transporters: Smf1p, Smf2p, and Smf3p, encoded by SMF1, SMF2, and SMF3, respectively. Here we report a comparative analysis of the yeast Smf proteins at the levels of localization, regulation, and function of the corresponding metal transporters. Smf1p and Smf2p function in cellular accumulation of manganese, and the two proteins are coregulated by manganese ions and the BSD2 gene product. Under manganese-replete conditions, Bsd2p facilitates trafficking of Smf1p and Smf2p to the vacuole, where these transport proteins are degraded. However, Smf1p and Smf2p localize to distinct cellular compartments under metal starvation: Smf1p accumulates at the cell surface, while Smf2p is restricted to intracellular vesicles. The third Nramp homologue, Smf3p, is quite distinctive. Smf3p is not regulated by Bsd2p or by manganese ions and is not degraded in the vacuole. Instead, Smf3p is down-regulated by iron through a mechanism that does not involve transcription or protein stability. Smf3p localizes to the vacuolar membrane independently of metal treatment, and yeast cells lacking Smf3p show symptoms of iron starvation. We propose that Smf3p helps to mobilize vacuolar stores of iron.

* Corresponding author. Mailing address: Johns Hopkins University School of Public Health, 615 N. Wolfe St., Room 7032, Baltimore, MD 21205. Phone: (410) 955-3029. Fax: (410) 955-0116. E-mail: vculotta{at}jhsph.edu.

dagger Present address: Digene Corporation, Gaithersburg, Md.

Molecular and Cellular Biology, November 2000, p. 7893-7902, Vol. 20, No. 21
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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