Evidence for continuous basel generation of GC-MAF: absence in infantile osteopetrosis and restoration after bone marrow transplant.

localization may be cell-cycle dependent. Mutations in FA patients represent an important resource for understanding both the function of the FA proteins and the molecular basis of the disease. For instance, the FA patient-derived FANCC-L554P mutant, which has a leucine to proline mutation at amino acid 554, fails to complement the mitomycin-C sensitivity of FA-C cell lines7 and is defective in FANCA binding.1 To evaluate the effect of the L554P mutation on the subcellular localization of the protein, we analyzed retrovirally infected HeLa cells by indirect immunofluorescence. Interestingly, the FANCC-L554P mutant, unlike the wild-type protein, was only observed in the cytoplasm of retrovirally infected HeLa cells (Fig 1C). On the basis of these results, we conclude that the wild-type FANCC protein is normally localized in both nucleus and cytoplasm. The absence of nuclear accumulation of the FANCC-L554P protein allows several conclusions. First, the nuclear localization of wild-type FANCC is not merely caused by the protein overexpression. Second, the functional activity of the FANCC protein requires nuclear localization. The FANCC-L554P mutation results in cytoplasmic localization and abolishes function. The missense mutation is likely to disrupt a domain of the protein required for its functional activity. The COOH-terminus domain of FANCC may be required for binding to FANCA or other protein subunits of the FA complex or for localization or stabilization of the protein in the nucleus.