Retinylidene protein  Retinylidene protein, is a family of proteins that use retinal as a chromophore for light reception. It is the molecular basis for a variety of light-sensing systems from phototaxis in flagellates to eyesight in animals. Retinylidene proteins include all forms of opsin and rhodopsin (in the broad sense). While rhodopsin in the narrow sense refers to a dim-light visual pigment found in vertebrates, usually on rod cells, "rhodopsin" in the broad sense (as used here) refers any molecule consisting of an opsin and a retinal chromophore in the ground state. When activated by light, the chromophore is isomerized, at which point the molecule as a whole is no longer rhodopsin, but a related molecule such as metarhodopsin. However, it remains a retinylidene protein. The chromophore then separates from the opsin, at which point the bare opsin is a retinylidene protein. Thus, the molecule remains a retinylidene protein throughout the phototransduction cycle. All rhodopsins consist of two building blocks, a protein moiety and a reversibly covalently bound non-protein cofactor, retinal (retinaldehyde). The protein structure of rhodopsin consists of a bundle of seven transmembrane helices that form an internal pocket binding the photoreactive chromophore. They form a superfamily with other membrane-bound receptors containing seven transmembrane domains, for example odor and chemokine receptors. Instead of being activated by binding chemical ligands like their relatives, rhodopsins contain retinal which changes conformation in reaction to light via photoisomerization and thus are activated by light. The retinal molecule can take on several different cis-trans isomeric forms, such as all-"trans", 11-"cis" and 13-"cis". Photoisomerization (light-dependent isomerization) of retinal from "cis" to "trans" or vice versa induces a conformational change in the receptor protein. This change acts as a molecular switch to activate a signal transduction mechanism within the cell. Depending on the type of rhodopsin, it either opens an ion channel (for example in bacteria) or activates an associated G protein and triggers a second messenger cascade (for example in animal eyes). Retinylidene proteins or rhodopsins are present in many species from bacteria to algae and animals.