There were errors published in J. Cell Sci. 123, 3447-3455. In Fig. 1, the arrows showing enzymatic reactions for MEC-17, HDAC6 and SIRT2 were pointing in the wrong direction. In addition, the C-terminal amino acid on the tail of -tubulin is Y (and not T). The correct figure is shown below. After publication of this Commentary, Shida and colleagues reported independent identification of MEC-17 homologs in diverse organisms as -tubulin acetyltransferases (Shida et al., 2010). Contrary to what we stated in our article and in the referenced paper (Akella et al., 2010), the newest version of the assembled genome of Chlamydomonas reinhardtii contains a sequence encoding an apparent homolog of the MEC-17 -tubulin acetyltransferase (locus ID Cre07.g345150).

Introductory ParagraphLong-lived microtubules endow the eukaryotic cell with long-range transport abilities. While long-lived microtubules are acetylated on lysine 40 of α-tubulin (αK40), acetylation takes place after stabilization1 and does not protect against depolymerization2. Instead, αK40 acetylation has been proposed to mechanically stabilize microtubules3. Yet how modification of αK40, a residue exposed to the microtubule lumen and inaccessible from MAPs and motors1,4, could affect microtubule mechanics remains an open question. Here we develop FRET-based assays that report on the lateral interactions between protofilaments and find that αK40 acetylation directly weakens inter-protofilament interactions. Congruently, αK40 acetylation affects two processes largely governed by inter-protofilament interactions, reducing the nucleation frequency and accelerating the shrinkage rate. Most relevant to the biological function of acetylation, microfluidics manipulations demonstrate that αK40 acetylation enhances flexibility and confers resilience against repeated mechanical stresses. Thus, unlike deacetylated microtubules that accumulate damages when subjected to repeated stresses, long-lived microtubules are protected from mechanical aging through their acquisition of αK40 acetylation. Thus, unlike other tubulin post-translational modifications that act through MAPs, motors and severing enzymes, intraluminal acetylation directly tunes the compliance and resilience of microtubules.

Eukaryotic cells rely on long-lived microtubules for intracellular transport and as compression-bearing elements. Intriguingly, long-lived microtubules are acetylated inside their lumen and microtubule acetylation has been proposed to modify microtubule mechanics. Here we found that tubulin acetylation is required for the mechanical stabilization of long-lived microtubules in cells. Depletion of the tubulin acetyltransferase TAT1 led to a significant increase in the frequency of microtubule breakage and nocodazole-resistant microtubules lost upon removal of acetylation were largely restored by either pharmacological or physical removal of compressive forces. In vitro reconstitution experiments demonstrated that acetylation is sufficient to protect microtubules from mechanical breakage. Thus, acetylation increases mechanical resilience to ensure the persistence of long-lived microtubules.

INCENP acts as a protein scaffold that integrates the functions of two crucial mitotic kinases, Aurora B and Polo, at centromeres of mitotic chromosomes.

Abbreviations used in this paper: BIR, baculovirus IAP repeat; CPC, chromosomal passenger protein complex; hSurvivin, human Survivin; IAP, inhibitor of apoptosis protein; INCENP, inner centromere protein; NES, nuclear export sequence; tTA, tetracycline transactivator.The online version of this article contains supplemental material.

INCENP fine tunes the level of aurora B kinase activity, which in turn correlates with different functional states of the chromosome passenger complex.

INCENP, Borealin, Survivin, and Aurora B kinase comprise the chromosomal passenger complex, an essential regulator of mitotic events. INCENP (inner centromere protein) binds and activates Aurora B through a feedback loop involving phosphorylation of a Thr-Ser-Ser (TSS) motif near the INCENP C terminus. Here, we have examined the role of the TSS motif in vertebrate cells using an DT40 INCENPON/OFF conditional knock-out cell line in which mutants are expressed in the absence of wild-type INCENP. Our analysis confirms that regulated phosphorylation of the two serine residues (presumably by Aurora B) is critical for full activation of the kinase and is essential for cell viability. Cells expressing INCENP mutants bearing either phospho-null (TAA) or phospho-mimetic (TEE) mutations exhibit significant levels of Aurora B kinase activity but fail to undergo normal spindle elongation or complete cytokinesis. This work confirms previous suggestions that INCENP can act as a rheostat, with different INCENP mutants promoting differing degrees of kinase activation. Our results also reveal that mitotic progression is accompanied by a requirement for progressively higher levels of Aurora B kinase activity.

Bone regeneration is a coordinated process involving the connection between blood vessels and bone cells. Glycoprotein non-metastatic melanoma protein B (GPNMB) is known to be vital in bone formation. However, the effect of GPNMB on bone regeneration and the underlying molecular mechanism are still undefined. Fibroblast growth factor receptor (FGFR)-mediating signaling is pivotal in bone formation and angiogenesis. Therefore, we assessed GPNMB function as a communicating molecule between osteoblasts and angiogenesis, and the possible correlation with FGFR-1 signaling. Recombinant GPNMB dose-dependently increased the differentiation of human bone marrow stromal cells (hBMSCs) into osteoblasts, as well as the mRNA levels of osteoblasts marker alkaline phosphatase (ALP) and osteocalcin (OCN). Furthermore, these increases depended on the activation of FGFR-1 signaling, as pretreatment with FGFR-1 siRNA or its inhibitor SU5402 dramatically dampened GPNMB-induced osteogenesis. Additionally, GPNMB triggered dose-dependently the proliferation and migration of human umbilical vein endothelial cells (hUVECs), FGFR-1 phosphorylation, as well as capillary tube and vessels formation in vitro and in vivo. Blocking FGFR-1 signaling dampened GPNMB-induced angiogenic activity. Following construction of a rodent cranial defect model, scaffolds delivering GPNMB resulted in an evident increase in blood vessels and new bone formation; however, combined delivery of GPNMB and SU5402 abated these increase in defect sites. Taken together, these results suggest that GPNMB stimulates bone regeneration by inducing osteogenesis and angiogenesis via regulating FGFR-1 signaling. Consequently, our findings will clarify a new explanation about how GPNMB induces bone repair, and provide a potential target for bone regeneration therapeutics and bone engineering.