Which type of protein binds a ligand on the cell surface or within cells, often initiating a downstream signaling cascade or altering gene transcription?

The type of protein that binds a ligand on the cell surface or within cells are transmembrane receptors. These proteins have 3 domains: an extracellular part that does the binding, a hydrophobic domain that extends through the membrane, and an intracellular region that transmits the signal. The most common membrane-anchored receptors are: G protein-coupled receptors, receptor tyrosine kinases, and ligand-gated ion channels.

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digestion

it excretes wastes