High-sensitivity sequencing of large proteins: partial structure of the rapamycin-FKBP12 target.

We report on studies leading to refinements of various steps of the protein internal sequencing process. Specifically, the developments comprise (1) higher-sensitivity chemical sequencing through background reduction; (2) improved peptide recovery from rapid in situ digests of nanogram amount, nitrocellulose-bound proteins; and (3) accurate UV spectroscopic identification of Trp- and Cys-containing peptides. In addition, we describe strategies for 2-dimensional liquid chromatographic peptide isolation from complex mixtures and a multi-analytical approach to peptide sequence analysis (Edman sequencing, matrix-assisted laser desorption mass spectrometry, and UV spectroscopy). Both strategies were applied in tandem to the primary structural analysis of a gel-purified, 250-kDa protein (mammalian target of rapamycin-FKBP12 complex), available in low picomolar quantities only. More than 300-amino acids worth of sequence was obtained in mostly uninterrupted stretches, several containing Trp, Cys, His, and Ser. That information has allowed the matching of a biological function of a mammalian protein to a yeast gene product with a well-characterized mutant phenotype. The results also demonstrate that extended chemical sequencing analysis (e.g., 26 successive amino acids) is now feasible, starting with initial yields well below 1 pmol.