Full‐Length Cryptochrome 1 in the Outer Segments of the Retinal Blue Cone Photoreceptors in Humans and Great Apes Suggests a Role Beyond Transcriptional Repression

Mammalian cryptochrome 1 (CRY1) is a central player in the circadian transcription-translation feedback loop, crucial for maintaining a roughly 24-h rhythm. CRY1 was suggested to also function as a blue-light photoreceptor in humans and has been found to be expressed at the mRNA level in various cell types of the inner retina. However, attempts to detect CRY1 at the protein level in the human retina have remained unsuccessful so far. Using various C-terminal specific antibodies recognizing full-length CRY1 protein, we consistently detected selective labeling in the outer segments of short wavelength-sensitive (SWS1, "blue") cone photoreceptor cells across human, bonobo, and gorilla retinae. No other retinal cell types were stained, which is in contrast to what would be expected of a ubiquitous clock protein. Subcellular fractionation experiments in transfected HEK cells using a C-terminal specific antibody located full-length CRY1 in the cytosol and membrane fractions. Our findings indicate that human CRY1 has several different functions including at least one nonclock function. Our results also raise the likely possibility that several different versions of CRY1 exist in humans. We suggest that truncation of the C-terminal tail, maybe to different degrees, may affect the localization and function of human CRY1.