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fig4: EB Residues Important for Growing Microtubule-End Recognition(A) Two views of conserved EB surface residues at the microtubule interface. The blue mesh depicts the surface of Mal3 CH domain residues found to be <5 Å away from tubulin residues in the pseudoatomic model. Almost all of the conserved CH domain surface residues found in the five EBs (spacefill atoms) form part of the contact surface. An especially large fraction of conserved residues (yellow spacefill) contact the β-tubulin H3 helix, whereas other conserved residues (blue spacefill) are part of the other tubulin contacts.(B) The identified Mal3-GTPγS microtubule interface provides a structural explanation for previous mutagenesis results obtained with EB1 (Slep and Vale, 2007) (left, front view; right, end-on view from the minus end). Previous mutations shown to disrupt plus-end tracking of EB1 in cells correspond to amino acids contacting the microtubule surface (red spacefill). Our structure shows that these patches are part of contact sites between the CH domain and β3- or β4-tubulin. In contrast, mutations without a noticeable effect are distant from the microtubule surface (green spacefill).See also Figure S3.
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).
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.
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