Protein kinase peptide substrate determination using peptide libraries

A method of identifying a compound that modulates the protein kinase catalyzed phosphorylation of a preferred peptide substrate, the method comprising:

• (a) incubating a protein kinase with a peptide library in the presence of kinase reaction components, the library comprising library members;
• (b) separating library members from the kinase reaction components using size exclusion chromatography to give a pool of phosphopeptides and unphosphorylated peptides;
• (c) contacting the pool with immobilized gallium ions to form chelated phosphopeptides;
• (d) eluting chelated phosphopeptides away from the gallium ions to give eluted phosphopeptides;
• (e) sequencing the eluted phosphopeptides to elucidate a preferred amino acid sequence of a preferred peptide substrate for a protein kinase;
• (f) incubating a protein kinase with a preferred peptide substrate comprising the preferred amino acid sequence in the presence and absence of a test modulator compound;
• and
• (g) determining an amount of preferred peptide substrate that is phosphorylated in the presence of the protein kinase, wherein a difference in phosphorylation of the preferred peptide substrate in the presence of the test modulator compound relative to phosphorylation of the preferred peptide substrate in the absence of the test modulator compound is indicative of modulation, whereby a compound that modulates the protein kinase catalyzed phosphorylation of a preferred peptide substrate is identified.

A method of designing a modulator for a protein kinase, the method comprising:

• (a) incubating a protein kinase with a peptide library in the presence of kinase reaction components, the library comprising library members;
• (b) separating library members from the kinase reaction components using size exclusion chromatography to give a pool of phosphopeptides and unphosphorylated peptides;
• (c) contacting the pool with immobilized gallium ions to form chelated phosphopeptides;
• (d) eluting chelated phosphopeptides away from the gallium ions to give eluted phosphopeptides;
• (e) sequencing the eluted phosphopeptides to elucidate a preferred amino acid sequence of a preferred peptide substrate for a protein kinase, wherein the preferred peptide substrate is phosphorylated;
• (f) quantifying in an assay for protein kinase activity a degree to which the phosphorylated preferred peptide substrate comprising a preferred amino acid sequence modulates the activity of the protein kinase;
• (g) selecting a chemical modification of the phosphorylated preferred peptide substrate comprising a preferred amino acid sequence wherein the interaction between the protein kinase and the phosphorylated preferred peptide substrate is predicted to be modulated by the chemical modification;
• (h) performing the chemical modification on the phosphorylated preferred peptide substrate;
• (i) contacting the modified phosphorylated preferred peptide substrate with the protein kinase;
• (j) quantifying in a biological assay for protein kinase activity a degree to which the modified phosphorylated preferred peptide substrate modulates the biological activity of the protein kinase;
• and
• (k) comparing the activity of the protein kinase in the presence of the modified phosphorylated preferred peptide substrate with the activity of the protein kinase in the absence of the modified phosphorylated preferred peptide substrate, whereby a modulator for a protein kinase is designed.

A method of designing a substrate for a protein kinase, the method comprising:

• (a) providing an unmodified preferred peptide substrate selected from the group consisting of SEQ ID NOs. 1-10;
• (b) quantifying phosphorylation of the unmodified preferred peptide substrate in an assay measuring protein kinase activity;
• (c) selecting a chemical modification of the preferred peptide substrate wherein an interaction between the protein kinase and the unmodified preferred peptide substrate is predicted to be modulated by the chemical modification;
• (d) performing the chemical modification on the unmodified preferred peptide substrate to form a modified preferred peptide substrate;
• (e) contacting the modified preferred peptide substrate with the protein kinase;
• (f) quantifying phosphorylation of the modified preferred peptide substrate, in an assay measuring protein kinase activity;
• (g) comparing the activities of the protein kinase in the presence of the modified preferred peptide substrate with the activity of the protein kinase in the presence of the unmodified preferred peptide substrate;
• and
• (h) repeating steps (c) through (g) a desired number of times to achieve a desired substrate property, whereby a substrate for a protein kinase is designed.

A method of elucidating a preferred amino acid sequence of a preferred peptide substrate for a protein kinase, the method comprising:

• (a) incubating a protein kinase with a peptide library in the presence of kinase reaction components, the library comprising library members having one or more variant positions;
• (b) separating library members from the kinase reaction components using size exclusion chromatography to give a pool of phosphopeptides and unphosphorylated peptides;
• (c) contacting the pool with immobilized gallium ions to form chelated phosphopeptides;
• (d) eluting chelated phosphopeptides away from the gallium ions to give eluted phosphopeptides;
• (e) sequencing the eluted phosphopeptides;
• (f) fixing a residue at one or more variant positions in a library member;
• and
• (g) repeating steps (a) through (d) a desired number of times, whereby a preferred amino acid sequence of a preferred peptide substrate for a protein kinase is elucidated.