3.4 Summary of Section 3

• Proteins are often the major components of signalling pathways and they have many different roles.
• Proteins can be modified in different ways to control their conformation, activity and interaction with other proteins. These modifications often act as switches to turn signalling processes on or off.
• Kinases phosphorylate other proteins, or themselves, on serine, threonine and tyrosine amino acid residues. Phosphatases remove phosphate groups from proteins. Cells express many different kinds of kinases and phosphatases that have specific protein substrates.
• G-proteins bind GDP or GTP. They are inactive when they have GDP bound, but become active when the GDP is exchanged for GTP. The exchange of GDP for GTP is activated by proteins that function as GEFs (guanine nucleotide-exchange factors). G-proteins hydrolyse GTP to GDP to terminate signalling. The GTPase activity is accelerated by proteins called GAPs (GTPase-activating proteins).
• The ability of a protein to interact with another is determined by conserved protein domains. These are sequences of amino acids that generate an interaction site for one protein to specifically bind to another.
• Many proteins have multiple domains. This allows them to have distinct binding partners and functions.
• Signalling pathways often involve the formation of complexes consisting of multiple proteins, each of which participate in a relay of information from receptor to effector.