Prelamin A is farnesylated and methylated on the cysteine residue of a carboxyl-terminal CaaX motif. In the nucleus, prelamin A is processed to lamin A by endoproteolytic removal of the final 18 amino acids, including the farnesylated cysteine residue. Using the yeast two-hybrid assay, we isolated a novel human protein, Narf, that binds the carboxyl-terminal tail of prelamin A. Narf has limited homology to iron-only bacterial hydrogenases and eukaryotic proteins of unknown function. Narf is encoded by a 2-kilobase mRNA expressed in all human cell lines and tissues examined. The protein is detected in the nuclear fraction of HeLa cell lysates on Western blots and can be extracted from nuclear envelopes with 0.5 M NaCl. When a FLAG epitope-tagged Narf is expressed in HeLa cells, it is exclusively nuclear and partially co-localizes with the nuclear lamina. The farnesylation status of prelamin A determines its ability to bind to Narf. Inhibition of farnesyltransferase and mutation or deletion of the CaaX motif from the prelamin A tail domain inhibits Narf binding in yeast two-hybrid and in vitro binding assays. The prenyl-dependent binding of Narf to prelamin A is an important first step in understanding the functional significance of the lamin A precursor.
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