Assembly of Signaling Complexes

Signaling complexes are protein clusters assembled on a receptor's intracellular docking sites that aid signal transduction by grouping molecules into a single location. This assembly ensures rapid and selective responses to extracellular signals by holding signaling proteins in close proximity, enabling efficient communication within the cell.

Scaffold proteins serve as docking sites for multiple protein partners in a signaling cascade, spacing them closely to reduce the time required for proteins to find their interacting partners. An activated membrane receptor sends messages to protein scaffolds to load with required signaling proteins, or signaling proteins may be pre-anchored to the scaffold before receiving a receptor message to increase efficiency.

SH2 and phosphotyrosine-binding (PTB) domains in docking proteins bind to phosphorylated tyrosines on receptors or other signaling proteins. These domains recognize specific sequences and residues near the phosphotyrosine, controlling the specificity and affinity of protein-protein interactions that organize signaling complexes.

SH3 domains bind to proline-rich sequences in signaling proteins, while pleckstrin homology (PH) domains recognize charged head groups of phosphoinositides on the plasma membrane. Together, these domains control the specificity of protein-protein and protein-phospholipid interactions that position signaling proteins within complexes.

Signaling complexes assemble through multiple mechanisms. Some signals induce phosphorylation of receptors or phosphoinositides in the plasma membrane, creating temporary docking sites. Alternatively, large scaffold proteins may pre-assemble signaling proteins on an inactive receptor, positioning them for rapid activation when the signal arrives.

Adaptor proteins such as Grb2 contain both SH2 and SH3 domains that link a single phosphotyrosine site to multiple intracellular relay proteins. This dual-domain architecture allows Grb2 to regulate both MAP kinase and PI3 kinase pathways, enabling amplification and diversification of signals through amplifying signals via enzymatic cascade mechanisms.

Scaffold proteins associated with G-protein-coupled receptors (GPCRs) interact through GPCR PDZ domains, which facilitate interactions with the carboxyl-termini of intracellular signaling proteins. Ligand binding causes receptor conformation changes that enhance scaffold protein interaction with G-proteins and GPCRs, with beta-arrestins being one of the best-characterized scaffold families.