The diagram is divided into three main sections. Section 1, labeled Microtubule tip stabilization, shows how proteins like CP110 and CSPP bind to the microtubule to enhance lateral interactions or prevent protofilament peeling. Section 2, labeled Microtubule growth inhibition, illustrates how proteins such as CEP104 and CPAP suppress longitudinal tubulin interactions, prevent tube closure, and promote protofilament peeling or tubulin removal. Section 3, labeled Facilitation of slow tubulin incorporation, depicts how proteins like CPAP, TOGARAM1, and CLASP facilitate the addition of tubulin to microtubule plus ends. The central image shows a microtubule with labeled sections indicating the different processes and proteins involved in regulating microtubule growth.
Model for the regulation of slow growth of centriolar and ciliary microtubules. Proposed model for regulation of slow-growing microtubule plus ends with three essential features: (1) stabilization of slowly growing plus ends by proteins like CP110 and CSPP (blue) that bind to the luminal side of the microtubule and may enhance lateral interactions between neighboring protofilaments (horizontal black arrows) or make the protofilaments straighter, preventing them from peeling (black arrow with a red cross). (2) Inhibition of growth by proteins such as CEP104 and the LID domain of CPAP (magenta) that may occlude the solvent-exposed surface of β-tubulin and partially suppress longitudinal tubulin interactions, by proteins such as CP110 that may inhibit growth by preventing tube closure, and by depolymerizing kinesins such as Klp10A (KIF2C/MCAK) and KIF24 that promote protofilament peeling at the microtubule plus end (curved black arrow) and KIF19A and KIF7 that can inhibit growth or depolymerize microtubules from the plus end. (3) Facilitation of slow microtubule growth by polymerases like CPAP and TOGARAM1 (green), which can bind soluble tubulin and promote its addition to microtubule plus ends with unfavorable configurations like blunt protofilaments or a small GTP cap (black arrow), or CLASP that facilitates growth by inhibiting catastrophes and promoting rescues.