Absolute proteomic quantification reveals design principles of sperm flagellar chemosensation

Cilia serve as cellular antennae that translate sensory information into physiological responses. In the sperm flagellum, a single chemoattractant molecule can trigger a Ca 2+ rise that controls motility. The mechanisms underlying such ultra‐sensitivity are ill‐defined. Here, we determine by mass spectrometry the copy number of nineteen chemosensory signaling proteins in sperm flagella from the sea urchin Arbacia punctulata . Proteins are up to 1,000‐fold more abundant than the free cellular messengers cAMP, cGMP, H + , and Ca 2+ . Opto‐chemical techniques show that high protein concentrations kinetically compartmentalize the flagellum: Within milliseconds, cGMP is relayed from the receptor guanylate cyclase to a cGMP‐gated channel that serves as a perfect chemo‐electrical transducer. cGMP is rapidly hydrolyzed, possibly via “substrate channeling” from the channel to the phosphodiesterase PDE5. The channel/PDE5 tandem encodes cGMP turnover rates rather than concentrations. The rate‐detection mechanism allows continuous stimulus sampling over a wide dynamic range. The textbook notion of signal amplification—few enzyme molecules process many messenger molecules—does not hold for sperm flagella. Instead, high protein concentrations ascertain messenger detection. Similar mechanisms may occur in other small compartments like primary cilia or dendritic spines. Synopsis Cellular sensory compartments serve highly specialised functions by means of a unique repertoire of signalling proteins. Here, quantitative protein profiling combined with structural analyses in sea urchin sperm flagella reveal reversed enzyme‐messenger ratios in chemosensory signalling, which ensure high‐sensitivity, low‐noise messenger detection. Absolute quantification of 11 chemosensory signaling proteins is applied to purified flagella of Arbacia punctulata . Determination of flagellar volume by cryo‐electron tomography allows for calculation of protein concentrations. Signaling proteins are orders of magnitude more abundant than free messengers. Opto‐chemical techniques show kinetic compartmentalisation of the flagellum at the cGMP‐gated channel/PDE5 tandem. Graphical Abstract Reversed enzyme‐messenger ratios in sperm cilia ensure high‐sensitivity, low noise messenger detection.