Figure 2. Phosphorylation events as double switches.

(A) A protein (black horizontal line) includes a segment that matches two sequence patterns: the first is typical for SH3 domain binding (green), and the second typifies SH2 domain binding (red). The non-phosphorylated form binds SH3 and not SH2 (upper), while phosphorylation inverts the binding preferences (lower). (B) Specificity switches within the PDZ domain family. A protein (black horizontal line) includes a segment that may bind distinct PDZ domains (upper). The non-phosphorylated form binds PDZ^a and not PDZ^b, while phosphorylation inverts these binding preferences (lower).