Fractionation of small tryptic phosphopeptides by alkaline PAGE followed by amino acid sequencing.

A novel two-step approach for localizing the site(s) of phosphorylation within intact proteins is described. Phosphorylated (32P-labeled) tryptic peptides are first resolved in a high-percentage polyacrylamide gel that has been optimized for the enrichment and separation of small, negatively charged peptides. Then the resolved peptides are located by autoradiography, excised, eluted and immobilized on a positively charged membrane, Immobilon -N, where they can be sequenced directly. The methods have been developed using a small, basic phosphoprotein (histone H1 from Tetrahymena); however, the approach is probably applicable to a wide variety of phosphoproteins.