The proteins of signaling pathways in eukaryotic cells selectively interact with one another and with small molecules such as phospholipids. These interactions frequently require modular domains that retain their capacity to recognize defined peptide motifs (or phospholipids) when they are expressed separately in cells. Signaling proteins often possess many of these domains and so are capable of multiple interactions with other proteins and phospholipids. Furthermore, domains within the same protein have the potential to interact with each other, providing a sophisticated means of switching signaling proteins on and off.
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