Sensing Proteins

Sensing proteins (receptors), to which specific signaling molecules

may attach, are embedded in the cell membrane (cell surface

receptors), or in the cytoplasm, or in the cell nucleus (nuclear

receptors) [Gether and Kobilka (1998); Ghanouni et al. (2001);

Kobilka and Deupi (2007)]. A molecule that binds to a receptor is

called a ligand and can be a peptide (short protein) or another small

molecule such as neurotransmitters, hormones, pharmaceutical

drugs, or toxins. A particular family of receptors is that of opsins,

which are sensitive to light, that is, photons [Bergo et al. (2004);

Dioumaev et al. (2002); Friedrich et al. (2002); Lozier et al. (1975);

Luecke (2000); Standfuss (2011)].