Splitting the ciliary axoneme: Implications for a “Switch‐Point” model of dynein arm activity in ciliary motion

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Abstract
In the presence of specific inhibitors of beat, 20 μM VO43− or pCa 4, mussel gill lateral (L) cilia can be arrested in two positions—“hands down” or “hands up”—at opposite ends of the stroke cycle. Cilia move to these positions by doublet microtubule sliding. Axonemes of arrested cilia, still tethered to the cell, are intact after demembranation and protease treatment. When reactivated by 4 mM ATP with inhibitors present, about 40% split apart. Splits are not random but occur preferentially between different specific doublets in the two opposite arrest positions. Several different related patterns of splitting are observed; for every pattern in “hands down” axonemes, there is a corresponding complementary split pattern in “hands up” axonemes. In some split patterns two doublets remain firmly attached to the central pair; these also differ depending on axonemal position. Although some of the patterns seen may be artifactual or difficult to explain, the complementary splitting patterns are predictable with simple assumptions by a “switch point” hypothesis of ciliary activity where, during each recovery stroke, doublets 6–8 have active dynein arms, while during each effective stroke, arms on doublets 1–4 become active, and arms 6–8 are turned off. Because of a difference between the patterns seen and the predictions, the status of the arms on doublet 9 is unresolved. The patterns also suggest that a spokecentral sheath attachment cycle may correlate with switching of arm activity during the generation of an asymmetric beat.