Rapid Protein Sequencing by Tandem Mass Spectrometry and cDNA Cloning of p20-CGGBP

The autonomous expansion of the unstable 5′-d(CGG) n -3′ repeat in the 5′-untranslated region of the humanFMR1 gene leads to the fragile X syndrome, one of the most frequent causes of mental retardation in human males. We have recently described the isolation of a protein p20-CGGBP that binds sequence-specifically to the double-stranded trinucleotide repeat 5′-d(CGG)-3′ (Deissler, H., Behn-Krappa, A., and Doerfler, W. (1996)J. Biol. Chem. 271, 4327–4334). We demonstrate now that the p20-CGGBP can also bind to an interrupted repeat sequence. Peptide sequence tags of p20-CGGBP obtained by nanoelectrospray mass spectrometry were screened against an expressed sequence tag data base, retrieving a clone that contained the full-length coding sequence for p20-CGGBP. A bacterially expressed fusion protein p20-CGGBP-6xHis exhibits a binding pattern to the double-stranded 5′-d(CGG) n -3′ repeat similar to that of the authentic p20-CGGBP. This novel protein lacks any overall homology to other known proteins but carries a putative nuclear localization signal. The p20-CGGBP gene is conserved among mammals but shows no homology to non-vertebrate species. The gene encoding the sequence for the new protein has been mapped to human chromosome 3.