Molecule of the Month: Phototropin

Scientists have known for many years that plants respond to light, growing toward sources of light and protecting themselves from light that is too harsh. In many of these cases, plants are responding to the level of blue light. Blue light has several advantages over other colors. It is among the most energetic of the visible colors, and thus can have a greater effect on chromophore molecules that absorb light. For plants that live in water, blue light also penetrates deeper than other colors. Plants, as well many other types of organisms, use sensor proteins to see the level of blue light and respond to it.

The light-sensing protein shown here is phototropin, found in many types of plants. It is important for responses that maximize the efficiency of photosynthesis, such as relocation of chloroplasts to optimal positions and expansion of leaves. Phototropin is composed of several domains. Two of these domains, termed "LOV" domains (short for "light, oxygen, or voltage"), hold flavin chromophores (shown in blue) that absorb blue light. These domains are connected to a serine-threonine kinase, which propagates the signal when the LOV domains are activated. There is not yet a structure of the entire protein, so the LOV domains are shown from PDB entries 2z6c and 4hhd , and the kinase domain is modeled on a similar kinase in PDB entry 1atp .

The light-sensing LOV domains are very modular, and scientists have engineered them in several ways, attaching them to other proteins and tuning the light that they absorb. For instance, the PDB includes a custom light sensor created by fusing a LOV domain with a different type of sensory kinase (PDB entry 4gcz ) and an engineered LOV domain that improves the fluorescence, for use in labeling molecules in living cells (PDB entry 4eeu ).

When the flavin chromophore absorbs light, the surrounding protein passes the signal along, ultimately changing how the plant will respond. For phototropin, the signal is passed to the kinase domain, which modifies other signaling proteins in the cell. The details are still under study, but some clues to the signaling process have been uncovered in the blue-light sensor Vivid, which is found in fungi. It is simpler than phototropin, with only a single LOV domain. In the dark, it is a monomer, shown here from PDB entry 2pd7 . When the chromophore absorbs light, it causes an alpha helix on the surface of the protein (shown in brighter colors here) to shift, causing the whole protein to dimerize, shown here from PDB entry 3rh8 .