Fig. (1).
Positive and negative roles of tyrosine phosphorylation in cell signaling. In this illustration, three tyrosine residues (Y1, Y2, Y3) on a protein may be subject to phosphorylation by a PTK. Phosphorylation of Y1 increases the activity of the protein. Phosphorylation of Y2 inhibits the activity of the protein. Phosphorylation of Y3 induces feedback inhibition such as recruitment of E3 ligase that causes degradation of the protein or GTPase Activator Protein (GAP) that turns off G-proteins. While dephosphorylation of Y1 by PTP1 inactivate the protein, dephosphorylation of Y2 and Y3 by PTP2 and PTP3 are necessary for sustained activity of the protein. Thus, PTP1 is a negative regulator whereas PTP2 and PTP3 are positive regulators that coordinately control the activity of the protein.