EBs recognize a nucleotide-dependent structural cap at growing microtubule ends.
Bottom Line: By binding close to the exchangeable GTP-binding site, the CH domain is ideally positioned to sense the microtubule's nucleotide state.The same microtubule-end region is also a stabilizing structural cap protecting the microtubule from depolymerization.This insight supports a common structural link between two important biological phenomena, microtubule dynamic instability and end tracking.
Affiliation: Cancer Research UK London Research Institute, Lincoln's Inn Fields Laboratories, 44 Lincoln's Inn Fields, London WC2A 3LY, UK.Show MeSH
Mentions: Mal3143 contact sites identified in our pseudoatomic model are conserved within α- and within β-tubulins from different species but not between α- and β-tubulins, explaining why the CH domain can distinguish between the two subunits of the tubulin heterodimer (Figure 3A). In addition, Mal3143 residues close to the microtubule surface (Figure 4A, blue mesh) match closely to conserved residues on the surface of its CH domain (Figure 4A, blue and yellow spacefill residues). These data strongly suggest that the structural basis of the recognition of growing microtubule ends by EBs is conserved. Our structure also shows that the microtubule-binding interface of the CH domain is much more extensive than suggested by a previous in vivo mutagenesis study (Slep and Vale, 2007), where deleterious mutations lie at two of the identified Mal3-tubulin interfaces (on β3- and β4-tubulin) (Figure 4B, red spacefill; Figure S3, red asterisks below alignment), whereas silent mutations lie away from these regions (Figure 4B, green spacefill; Figure S3, green asterisks below alignment). In particular, our model reveals that as well as additional contacts with two α-tubulins (α1 and α2), Mal3143 also contacts β3-tubulin at lower radius on the H3 helix, which, strikingly, is directly connected to the exchangeable nucleotide site (E site, Figure 3B).
Affiliation: Cancer Research UK London Research Institute, Lincoln's Inn Fields Laboratories, 44 Lincoln's Inn Fields, London WC2A 3LY, UK.