MFN1 structures reveal nucleotide-triggered dimerization critical for mitochondrial fusion.

Mitochondria are double-membraned organelles with variable shapes influenced by metabolic conditions, developmental stage, and environmental stimuli. Their dynamic morphology is a result of regulated and balanced fusion and fission processes. Fusion is crucial for the health and physiological functions of mitochondria, including complementation of damaged mitochondrial DNAs and the maintenance of membrane potential. Mitofusins are dynamin-related GTPases that are essential for mitochondrial fusion. They are embedded in the mitochondrial outer membrane and thought to fuse adjacent mitochondria via combined oligomerization and GTP hydrolysis. However, the molecular mechanisms of this process remain unknown. Here we present crystal structures of engineered human MFN1 containing the GTPase domain and a helical domain during different stages of GTP hydrolysis. The helical domain is composed of elements from widely dispersed sequence regions of MFN1 and resembles the 'neck' of the bacterial dynamin-like protein. The structures reveal unique features of its catalytic machinery and explain how GTP binding induces conformational changes to promote GTPase domain dimerization in the transition state. Disruption of GTPase domain dimerization abolishes the fusogenic activity of MFN1. Moreover, a conserved aspartate residue trigger was found to affect mitochondrial elongation in MFN1, probably through a GTP-loading-dependent domain rearrangement. Thus, we propose a mechanistic model for MFN1-mediated mitochondrial tethering, and our results shed light on the molecular basis of mitochondrial fusion and mitofusin-related human neuromuscular disorders.

[Pharmacokinetics of gypenoside XVâ ¡ in rats].

In this study, we established an HPLC-MS method to determine gypenoside XVⅡ in biosamples. The methodology results indicated that the linear range was 1-2 500 µg•L⁻¹ (r=0.996 3); intraday RSD values for high, medium and low concentrations were 9.9%, 3.0% 1.7%; interday RSD values were 16%, 14%, 2.5%; matrix effect ranged between 90.0%-100%, with RSD<15%. The recovery was more than 80.0%, with precision and accuracy in line with request. After the rats were orally and intravenously administered with gypenoside XVⅡ, the concentrations of gypenoside XVⅡ in plasma were determined, and pharmacokinetic parameter was calculated using pharmacokinetic software DAS 2.0. According to the main pharmacokinetic parameters of gypenoside XVⅡ, tmax was 0.17-0.20 h, t1/2 was 1.94-2.56 h, bioavailability of oral administration was 1.87%. The results indicated that the pharmacokinetic profiles of gypenoside XVⅡ were rapid absorption and distribution after oral administration, short time to peak and rapid elimination.

Aurora kinase A-mediated phosphorylation triggers structural alteration of Rab1A to enhance ER complexity during mitosis.

Morphological rearrangement of the endoplasmic reticulum (ER) is critical for metazoan mitosis. Yet, how the ER is remodeled by the mitotic signaling remains unclear. Here, we report that mitotic Aurora kinase A (AURKA) employs a small GTPase, Rab1A, to direct ER remodeling. During mitosis, AURKA phosphorylates Rab1A at Thr75. Structural analysis demonstrates that Thr75 phosphorylation renders Rab1A in a constantly active state by preventing interaction with GDP-dissociation inhibitor (GDI). Activated Rab1A is retained on the ER and induces the oligomerization of ER-shaping protein RTNs and REEPs, eventually triggering an increase of ER complexity. In various models, from Caenorhabditis elegans and Drosophila to mammals, inhibition of Rab1AThr75 phosphorylation by genetic modifications disrupts ER remodeling. Thus, our study reveals an evolutionarily conserved mechanism explaining how mitotic kinase controls ER remodeling and uncovers a critical function of Rab GTPases in metaphase.