Chaperonin TRiC bridges radial spokes for folding proteins locally translated in mammalian sperm flagella

As animals evolved from external to internal fertilization, sperm flagella, which once transiently propelled sperm in water to reach nearby eggs, developed to beat for days or even longer within the female reproductive tract. How flagella were remodeled accordingly remains unclear. Unlike externally fertilizing zebrafish and sea urchin, mammalian sperm flagella feature a unique barrel-shaped bridge between axonemal radial spokes RS1 and RS2, though its components and function remain unknown. Here, using cryo-EM and cryo-ET combined with mass spectrometry and immunofluorescence staining, we reveal that this RS1-RS2 barrel (RRB) in mammalian sperm flagella is a group II chaperonin TRiC/CCT. We purified and resolved the unforeseen cryo-EM structure of TRiC from bovine sperm flagella, discovering that flagella TRiC contains the testis-specific CCT6B subunit. Additionally, we resolved the cryo-ET map of mouse sperm flagella RSs, containing the RRB, achieving unprecedented local resolution of 10-14 [A]. The in-situ RRB map displays two extra densities within its chambers in a polarized manner, suggesting the presence of folding substrates and a co-chaperone (likely phosducin-like 2). Notably, mammalian flagella also contain components of translation machineries and actively synthesize proteins. Our results suggest that the RRB TRiC folds locally translated proteins to sustain mammalian flagella, providing new insights into flagellar remodeling in internally fertilizing species. Our findings also shed lights on male fertility and potential treatments for infertility.