Figure 1.
Figure 1. Schematic models showing the general organization of cilia and flagella and a model for the conversion of MTD sliding to bending. (A) Longitudinal view of an MTD with three 96-nm repeats shown in front view, i.e., looking from a neighboring B-tubule toward the A-tubule with its attached IDAs and ODAs. This is the classical view of the DRC density seen in previous EM studies (Mastronarde et al., 1992; Gardner et al., 1994). Red arrows indicate the orientation of additional views used in our figures: “view from bottom” looks at the 96-nm repeat from the center of the axoneme outwards, “from top” looks from the ODAs toward the axoneme center, “from prox.” is viewed from the flagellar base to its tip, and “from distal” is vice versa. (B) Simplified cross-sectional view of an axoneme with nine outer MTDs surrounding the CP complex (CPC); the viewing direction is from the flagellar tip (from distal). Three structures connect neighboring MTDs, the ODAs and IDAs and the nexin link (also called the circumferential link). One MTD is boxed in red and shown in longitudinal view in A. The blue and green boxes each highlight a pair of neighboring doublets from opposite sides of the axoneme. (C and D) Restricted interdoublet sliding model for ciliary and flagellar bend formation (Satir, 1968; Summers and Gibbons, 1971). Two pairs of MTDs from opposite sides of the axoneme, as highlighted in B, are shown. The dynein arms are anchored on the cargo MTD in an ATP-independent manner and walk toward the minus end (−) of the track MTD in an ATP-dependent manner, which causes sliding between neighboring MTDs. The links between MTDs are thought to be important for restricting and transforming MT sliding into bending. Note that the minus end–directed dynein motors (red) have to be active and produce MT sliding on alternate sides of the axoneme (switching between the blue and green side) to generate the effective and reverse bend (with opposite bending directions; black arrows). Panel A is adapted from Porter and Sale (2000). Panel B is adapted from Nicastro et al. (2006) with permission from Science.

Schematic models showing the general organization of cilia and flagella and a model for the conversion of MTD sliding to bending. (A) Longitudinal view of an MTD with three 96-nm repeats shown in front view, i.e., looking from a neighboring B-tubule toward the A-tubule with its attached IDAs and ODAs. This is the classical view of the DRC density seen in previous EM studies (Mastronarde et al., 1992; Gardner et al., 1994). Red arrows indicate the orientation of additional views used in our figures: “view from bottom” looks at the 96-nm repeat from the center of the axoneme outwards, “from top” looks from the ODAs toward the axoneme center, “from prox.” is viewed from the flagellar base to its tip, and “from distal” is vice versa. (B) Simplified cross-sectional view of an axoneme with nine outer MTDs surrounding the CP complex (CPC); the viewing direction is from the flagellar tip (from distal). Three structures connect neighboring MTDs, the ODAs and IDAs and the nexin link (also called the circumferential link). One MTD is boxed in red and shown in longitudinal view in A. The blue and green boxes each highlight a pair of neighboring doublets from opposite sides of the axoneme. (C and D) Restricted interdoublet sliding model for ciliary and flagellar bend formation (Satir, 1968; Summers and Gibbons, 1971). Two pairs of MTDs from opposite sides of the axoneme, as highlighted in B, are shown. The dynein arms are anchored on the cargo MTD in an ATP-independent manner and walk toward the minus end (−) of the track MTD in an ATP-dependent manner, which causes sliding between neighboring MTDs. The links between MTDs are thought to be important for restricting and transforming MT sliding into bending. Note that the minus end–directed dynein motors (red) have to be active and produce MT sliding on alternate sides of the axoneme (switching between the blue and green side) to generate the effective and reverse bend (with opposite bending directions; black arrows). Panel A is adapted from Porter and Sale (2000). Panel B is adapted from Nicastro et al. (2006) with permission from Science.

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