Genetically Separable Functions of the MEC-17 Tubulin Acetyltransferase Affect Microtubule Organization

Abstract

Background Microtubules (MTs) are formed from the lateral association of 11–16 protofilament chains of tubulin dimers, with most cells containing 13-protofilament (13-p) MTs. How these different MTs are formed is unknown, although the number of protofilaments may depend on the nature of the α- and β-tubulins. Results Here we show that the enzymatic activity of the C. elegans α-tubulin acetyltransferase (α-TAT) MEC-17 allows the production of 15-p MTs in the touch receptor neurons (TRNs) MTs. Without MEC-17, MTs with between 11 and 15 protofilaments are seen. Loss of this enzymatic activity also changes the number and organization of the TRN MTs and affects TRN axonal morphology. In contrast, enzymatically inactive MEC-17 is sufficient for touch sensitivity and proper process outgrowth without correcting the MT defects. Thus, in addition to demonstrating that MEC-17 is required for MT structure and organization, our results suggest that the large number of 15-p MTs, normally found in the TRNs, are not essential for mechanosensation. Conclusion These experiments reveal a specific role for α-TAT in the formation of MTs and in the production of higher order MTs arrays. In addition our results indicate that the α-TAT protein has functions that require acetyltransferase activity (such as the determination of protofilament number) and others that do not (presence of internal MT structures).

Crystal Structures of Tubulin Acetyltransferase Reveal a Conserved Catalytic Core and the Plasticity of the Essential N Terminus

“…One hypothesis is that this modification leads to subtle changes in the microtubule lattice that can affect binding of effectors and polymer dynamics. Consistent with this, ultrastructural analyses of microtubules in touch receptor neurons of mec-17 mutants show a variability in protofilament number when compared with wild-type and large lattice defects that cause microtubules to splay apart and compress radially (17,18).…”

“…In the case of TAT, the microtubule lattice could be functioning as the chaperone stabilizing its catalytically active conformation. Different organisms and cell types have microtubules with variable protofilament numbers, and TAT has evolved to function with these varying polymer architectures (17). This functional plasticity could be related to the structural plasticity that we have uncovered.…”

Molecular basis of the acetyltransferase activity of MEC-17 towards α-tubulin

“…Therefore, some other residue at the catalytic site in DrMEC-17 should act as a general base. Asp151 on the β7-α6 loop is in the vicinity of the acetyl group of acetyl-CoA, and mutation of Asp151 of DrMEC-17, the equivalent Asp157 of H. sapiens MEC-17 or Asp144 of C. elegans MEC-17 to Asn, resulted in abolishment of the enzymatic activity [4,10] ( Figure 1D), indicating that this highly conserved Asp might be the general base. In addition, the side chain of Phe177, a strictly conserved residue on strand β8, flanks the left side of Asp151 to create a locally hydrophobic environment for Asp151 ( Figure 1B), which might raise the pKa of Asp151 to facilitate the proton extraction of the substrate lysine.…”

“…Previously, the Chalfie group reported two missense mutations in allele of C. elegans MEC-17, u265, one of which (Leu46) corresponds to Gln53 of DrMEC-17 [9]. Intriguingly, although the acetyltransferase activity of MEC-17 is not required for touch sensation [10], the mec-17 (u265) worms are not touch sensitive [9], indicating that Leu46 of C. elegans MEC-17 and probably the equivalent Gln53 of DrMEC-17 might contribute to the substrate binding.…”

αTAT1 is the major α-tubulin acetyltransferase in mice

“…In Tetrahymena, this in turn slows the growth of cells treated with microtubule depolymerizing agents and increases the rate of depolymerization of axonemes 17 . In C. elegans, deletion of aTAT1 orthologues MEC17 and aTAT-2, reduces touch sensitivity 9,17 and disrupts microtubule structure and organization in touch receptor neurons 18,19 . Importantly, these effects depend upon both the acetyltransferase activity of a-TAT1 and on other as yet unknown functions of this protein 18 .…”