In a basic model of signal transduction, a signalling molecule binds to a specific receptor, and this activates a sequence (or web) of intracellular signalling molecules that spread the information to relevant parts of the cell, activating target molecules, which effect a cellular response.
Signalling between cells can be contact dependent or via secreted signalling molecules. The latter comprise paracrine, autocrine, endocrine or electrical signalling.
There are four types of cell surface receptors: ion channel receptors, 7-helix transmembrane receptors, receptors with intrinsic enzymatic activity, and enzyme-associated (recruiter) receptors. Receptors with intrinsic transcriptional activity are mostly intracellular.
Two basic categories of signalling molecules intervene in signal transduction, according to the spatial and temporal requirements of the signalling pathway.
a.Small diffusible signalling molecules (‘second messengers’) enable rapid signal amplification and a widespread cellular response.
b.Signalling proteins fulfil many roles (by virtue of protein–protein interaction and protein regulation), including signal integration, modulation, transduction and anchoring functions.
G proteins and proteins activated by phosphorylation on tyrosine, serine and/or threonine residues can act as molecular switches.
The subcellular location of the signalling protein is critical to its function, and this is aided by transient or preassembled signalling complexes.
Specific binding of signalling proteins to each other is critical for the effective transduction of the signal. Binding domains allow transient binding to specific (often phosphorylated) amino acid sequences or to phospholipids.