Function and molecular mechanism of N-terminal acetylation in autophagy.

Acetyl ligation to the amino acids in a protein is an important posttranslational modification. However, in contrast to lysine acetylation, N-terminal acetylation is elusive in terms of its cellular functions. Here, we identify Nat3 as an N-terminal acetyltransferase essential for autophagy, a catabolic pathway for bulk transport and degradation of cytoplasmic components. We identify the actin cytoskeleton constituent Act1 and dynamin-like GTPase Vps1 (vacuolar protein sorting 1) as substrates for Nat3-mediated N-terminal acetylation of the first methionine. Acetylated Act1 forms actin filaments and therefore promotes the transport of Atg9 vesicles for autophagosome formation; acetylated Vps1 recruits and facilitates bundling of the SNARE (soluble N-ethylmaleimide-sensitive factor activating protein receptor) complex for autophagosome fusion with vacuoles. Abolishment of the N-terminal acetylation of Act1 and Vps1 is associated with blockage of upstream and downstream steps of the autophagy process. Therefore, our work shows that protein N-terminal acetylation plays a critical role in controlling autophagy by fine-tuning multiple steps in the process.

Microscopy-based Saccharomyces cerevisiae complementation model reveals functional conservation and redundancy of N-terminal acetyltransferases.

N-terminal acetylation is a highly abundant protein modification catalyzed by N-terminal acetyltransferases (NATs) NatA-NatG. The Saccharomyces cerevisiae protein Arl3 depends on interaction with Sys1 for its localization to the Golgi and this targeting strictly requires NatC-mediated N-terminal acetylation of Arl3. We utilized the Arl3 acetylation-dependent localization phenotype as a model system for assessing the functional conservation and in vivo redundancy of several human NATs. The catalytic subunit of human NatC, hNaa30 (Mak3), restored Arl3 localization in the absence of yNaa30, but only in the presence of either yeast or human Naa35 subunit (Mak10). In contrast, hNaa35 was not able to replace its yeast orthologue without the co-expression of hNaa30, suggesting co-evolution of the two NatC subunits. The most recently discovered and organellar human NAT, NatF/Naa60, restored the Golgi localization of Arl3 in the absence of yNaa30. Interestingly, this was also true for hNaa60 lacking its membrane-binding domain whereas hNaa50 did not complement NatC function. This in vivo redundancy reflects NatC and NatF´s overlapping in vitro substrate specificities. The yeast model presented here provides a robust and rapid readout of NatC and NatF activity in vivo, and revealed evolutionary conservation of the NatC complex and redundancy between NatC and NatF.

Protein N-terminal acetylation is required for embryogenesis in Arabidopsis.

Early embryonic development generates precursors of all major cell types in Arabidopsis. Among these precursors, the hypophysis divides asymmetrically to form the progenitors of the quiescent center and columella stem cells. A great deal has been learnt about the mechanisms that control the asymmetric division of the hypophysis and embryogenesis at the transcriptional level; however, no evidence of regulation at the co- or post-translational level has been reported. Here, we show that mutation of the catalytic subunit (Naa10) or auxiliary subunit (Naa15) of NatA, an N-terminal acetyltransferase that catalyzes protein N-terminal acetylation, produces an embryo-lethal phenotype. In addition, Naa10 and Naa15 were found to interact physically in planta Further analysis revealed that the observed embryonic patterning defects started at the early globular stage and that the asymmetric division of the hypophysis was irregular; thus, no quiescent center progenitor cells were generated in naa10 and naa15 embryos. We further observed that the polar distributions of auxin and its efflux carrier PIN1 were disturbed in naa10 embryos. Our results suggest that NatA is required for asymmetric division of the hypophysis and early embryonic patterning in Arabidopsis, and provides a link between protein N-terminal acetylation and embryogenesis in plants.

NAA10 controls osteoblast differentiation and bone formation as a feedback regulator of Runx2.

Runt-related transcription factor 2 (Runx2) transactivates many genes required for osteoblast differentiation. The role of N-α-acetyltransferase 10 (NAA10, arrest-defective-1), originally identified in yeast, remains poorly understood in mammals. Here we report a new NAA10 function in Runx2-mediated osteogenesis. Runx2 stabilizes NAA10 in osteoblasts during BMP-2-induced differentiation, and NAA10 in turn controls this differentiation by inhibiting Runx2. NAA10 delays bone healing in a rat calvarial defect model and bone development in neonatal mice. Mechanistically, NAA10 acetylates Runx2 at Lys225, and this acetylation inhibits Runx2-driven transcription by interfering with CBFß binding to Runx2. Our study suggests that NAA10 acts as a guard ensuring balanced osteogenesis by fine-tuning Runx2 signalling in a feedback manner. NAA10 inhibition could be considered a potential strategy for facilitating bone formation.

Age-related changes in melatonin release in the murine distal colon.

Constipation and fecal impaction are conditions of the bowel whose prevalence increases with age. Limited information is known about how these conditions manifest; however, functional deficits are likely to be due to changes in signaling within the bowel. This study investigated the effects of age on colonic mucosal melatonin (MEL) release and the consequences this had on colonic motility. Electrochemical measurements of MEL overflow demonstrated that both basal and mechanically stimulated MEL release decreased with age. The MEL/serotonin also decreased with increasing age, and the trend was similar to that of MEL overflow, suggestive that age-related changes were primarily due to a reduction in MEL levels. Levels of N-acetylserotonin and the N-acetylserotonin/serotonin ratio were reduced with age, providing an explanation for the reduction in MEL release. Decreases in colonic motility were observed in animals between 3 and 24 months old. Exogenous application of MEL could reverse this deficit in aged colon. In summary, we propose that the age-related decline in MEL release may be due to either decreases or alterations in mechanosensory channels and/or a loss in levels/activity of the N-acetyltransferase enzyme responsible for the synthesis of N-acetylserotonin. Decreases in MEL release may explain the decreases in colonic motility observed in 24 month old animals and could offer a new potential therapeutic treatment for age-related constipation.