The primary cilium transduces extracellular signals into cellular responses in the cytoplasm through poorly understood mechanisms. During fertilisation in the bi-ciliated green alga Chlamydomonas reinhardtii, ciliary adhesion between the SAG1 adhesion receptor on plus cilia and SAD1 on minus cilia results in a near-immediate and sustained ∼10-fold increase of cellular cAMP to prepare the gametes for fertilisation. In this study (Awasthi et al., 2022), the authors use Chlamydomonas to investigate cilium-to-cytoplasm communication. First, by screening Chlamydomonas strains with mutations in genes specifically expressed in gametes, the authors identify the gene GSPK, encoding Gamete Specific Protein Kinase (GSPK), as being essential for efficient gamete fusion. Although the mutant gspk gametes were identical to wild-type gametes in ciliary adhesion and in their intraciliary biochemical responses to adhesion, fusion was impaired because cellular cAMP levels increased only transiently and less than two-fold on mixing of gspk(–) and gspk(+) gametes. Fusion was restored by addition of exogenous cAMP. For wild-type gametes, the authors show that GSPK is present in the cytoplasm and not the cilia, and that all GSPK is phosophorylated within 1 min after gamete mixing. Exogenous application of cAMP did not result in phosphorylation. Thus, their findings indicate that a still-unknown ciliary signal is necessary to generate a cytoplasmic response.
