A photoreceptor going nowhere but the nucleus.

Cryptochrome 2 (CRY2) is a blue/UV-A light receptor that regulates light inhibition of cell elongation and photoperiodic promotion of floral initiation in Arabidopsis. We and others have previously shown that CRY2 is a nuclear protein that regulates gene expression to affect plant development. We also showed that CRY2 is phosphorylated in response to blue light and the phosphorylated CRY2 is most likely active and degraded in blue light. Given that protein translation (and probably chromophore attachment) takes place in the cytosol and that a photoreceptor would absorb photon instantaneously, it would be interesting to know where those inter-connected events occur in the cell. Our results showed that freshly synthesized CRY2 photoreceptor is inactive in the cytosol although it may be photon-excited, it is imported into the nucleus where the photoreceptor is phosphorylated, performs its function, becomes ubiquitinated, and eventually gets degraded (Fig. 1).1 To our knowledge, this is the first example in any organism that a photoreceptor is shown to complete its post-translational life cycle in a single subcellular compartment.