Anti-CaM antibodies precipitate four polypeptides, one of which exhibits calcium-sensitive CaM binding. (A) Diagram of the central apparatus and a single doublet microtubule with associated structures. Central pair projections are labeled. Inserted table lists WT and mutant strains used in this study along with the associated structural defects. (B) CaM gel overlay of WT, pf14, and pf18 axonemes in high and low calcium conditions. CaM binds to a polypeptide of ∼110 kD specifically in the presence of calcium (red arrowhead); this protein is missing from pf18 axonemes. (C) Silver-stained gels of anti-CaM immunoprecipitation experiments (IPs) performed in low and high calcium buffers from axonemal extracts isolated from WT and mutant axonemes. Four polypeptides are precipitated that are highly enriched in high calcium IPs (HC1–4). These four polypeptides are missing or reduced from pf18 and pf16 anti-CaM IPs (blue asterisks) and are present at WT levels in pf6, cpc1, and pf14 anti-CaM IPs (red asterisks). These results tentatively localize HC1–4 to the C1c or C1d projections of the central apparatus. (D) Corresponding CaM gel overlays of high calcium anti-CaM IPs from WT and mutant axonemal extracts. Only one of the precipitated proteins, HC4, exhibits calcium-sensitive CaM binding.