Rab4：细胞囊泡循环运输过程的重要因子

囊泡运输是真核细胞中物质运输及信息交流的重要形式,Rab蛋白在这个过程中发挥着重要功能.Rab4是Rab蛋白家族的成员之一,参与调控早期内体的分选与内体循环途径.Rab4包括Rab4A、Rab4B和Rab4C 3个亚型.本文主要阐述了Rab4的结构特征、主要的效应蛋白和参与运输的货物蛋白以及影响细胞自噬、葡萄糖摄取、神经调节、心脏功能及肿瘤发生方面的功能.     

Regulation of cytokinesis by membrane trafficking involving small GTPases and the ESCRT machinery

During cell division, cells undergo membrane remodeling to achieve changes in their size and shape. In addition, cell division entails local delivery and retrieval of membranes and specific proteins as well as remodeling of cytoskeletons, in particular, upon cytokinetic abscission. Accumulating lines of evidence highlight that endocytic membrane removal from and subsequent membrane delivery to the plasma membrane are crucial for the changes in cell size and shape, and that trafficking of vesicles carrying specific proteins to the abscission site participate in local remodeling of membranes and cytoskeletons. Furthermore, the endosomal sorting complex required for transport (ESCRT) machinery has been shown to play crucial roles in cytokinetic abscission. Here, the author briefly overviews membrane-trafficking events early in cell division, and subsequently focus on regulation and functional significance of membrane trafficking involving Rab11 and Arf6 small GTPases in late cytokinesis phases and assembly of the ESCRT machinery in cytokinetic abscission.     

Thyreophoran vertebrae from the Callovian (Middle Jurassic) of the Vaches Noires cliffs (Normandy,France), with remarks on the dinosaur assemblage from the Vaches Noires

Two associated incomplete thyreophoran dorsal vertebrae from the Callovian Marnes de Dives Formation of the Vaches Noires cliffs, on the Normandy coast, are referred to an indeterminate stegosaur that appears to be different from Lexovisaurus, previously reported from the Callovian of western France. These vertebrae are the first evidence of thyreophorans from the Vaches Noires and complement the dinosaur assemblage from this locality, which hitherto consisted of several theropod taxa and an indeterminate sauropod. The dinosaur record from the Vaches Noires is heavily dominated by theropods and this imbalance is difficult to explain. A possible explanation may be that the dinosaur sample from the Vaches Noires is too small to be statistically significant and representative of the original faunal assemblage from which it is derived.     

Invasion by activated macrophages requires delivery of nascent membrane‐type‐1 matrix metalloproteinase through late endosomes/lysosomes to the cell surface

Macrophage migration into injured or infected tissue is a key aspect in the pathophysiology of many diseases where inflammation is a driving factor. Membrane‐type‐1 matrix metalloproteinase (MT1‐MMP) cleaves extracellular matrix components to facilitate invasion. Here we show that, unlike the constitutive MT1‐MMP surface recycling seen in cancer cells, unactivated macrophages express low levels of MT1‐MMP. Upon lipopolysaccharide (LPS) activation, MT1‐MMP synthesis dramatically increases 10‐fold at the surface by 15 hours. MT1‐MMP is trafficked from the Golgi complex to the surface via late endosomes/lysosomes in a pathway regulated by the late endosome/lysosome R‐SNAREs VAMP7 and VAMP8. These form two separate complexes with the surface Q‐SNARE complex Stx4/SNAP23 to regulate MT1‐MMP delivery to the plasma membrane. Loss of either one of these SNAREs leads to a reduction in surface MT1‐MMP, gelatinase activity and reduced invasion. Thus, inhibiting MT1‐MMP transport through this pathway could reduce macrophage migration and the resulting inflammation.     

Human MICAL1: Activation by the small GTPase Rab8 and small‐angle X‐ray scattering studies on the oligomerization state of MICAL1 and its complex with Rab8

Human MICAL1 is a member of a recently discovered family of multidomain proteins that couple a FAD‐containing monooxygenase‐like domain to typical protein interaction domains. Growing evidence implicates the NADPH oxidase reaction catalyzed by the flavoprotein domain in generation of hydrogen peroxide as a second messenger in an increasing number of cell types and as a specific modulator of actin filaments stability. Several proteins of the Rab families of small GTPases are emerging as regulators of MICAL activity by binding to its C‐terminal helical domain presumably shifting the equilibrium from the free – auto‐inhibited – conformation to the active one. We here extend the characterization of the MICAL1–Rab8 interaction and show that indeed Rab8, in the active GTP‐bound state, stabilizes the active MICAL1 conformation causing a specific four‐fold increase of k_cat of the NADPH oxidase reaction. Kinetic data and small‐angle X‐ray scattering (SAXS) measurements support the formation of a 1:1 complex between full‐length MICAL1 and Rab8 with an apparent dissociation constant of approximately 8 μM. This finding supports the hypothesis that Rab8 is a physiological regulator of MICAL1 activity and shows how the protein region preceding the C‐terminal Rab‐binding domain may mask one of the Rab‐binding sites detected with the isolated C‐terminal fragment. SAXS‐based modeling allowed us to propose the first model of the free full‐length MICAL1, which is consistent with an auto‐inhibited conformation in which the C‐terminal region prevents catalysis by interfering with the conformational changes that are predicted to occur during the catalytic cycle.     

How pollen tubes grow

Sexual reproduction of flowering plants depends on delivery of the sperm to the egg, which occurs through a long, polarized projection of a pollen cell, called the pollen tube. The pollen tube grows exclusively at its tip, and this growth is distinguished by very fast rates and reaches extended lengths. Thus, one of the most fascinating aspects of pollen biology is the question of how enough cell wall material is produced to accommodate such rapid extension of pollen tube, and how the cell wall deposition and structure are regulated to allow for rapid changes in the direction of growth. This review discusses recent advances in our understanding of the mechanism of pollen tube growth, focusing on such basic cellular processes as control of cell shape and growth by a network of cell wall-modifying enzymes, molecular motor-mediated vesicular transport, and intracellular signaling by localized gradients of second messengers.     

Functional involvement of TMF/ARA160 in Rab6-dependent retrograde membrane traffic

The small GTPase Rab6 regulates retrograde membrane traffic from endosomes to the Golgi apparatus and from the Golgi to the endoplasmic reticulum (ER). We examined the role of a Rab6-binding protein, TMF/ARA160 (TATA element modulatory factor/androgen receptor-coactivator of 160 kDa), in this process. High-resolution immunofluorescence imaging revealed that TMF signal surrounded Rab6-positive Golgi structures and immunoelectron microscopy revealed that TMF is concentrated at the budding structures localized at the tips of cisternae. The knockdown of either TMF or Rab6 by RNA interference blocked retrograde transport of endocytosed Shiga toxin from early/recycling endosomes to the trans-Golgi network, causing missorting of the toxin to late endosomes/lysosomes. However, the TMF knockdown caused Rab6-dependent displacement of N-acetylgalactosaminyltransferase-2 (GalNAc-T2), but not beta1,4-galactosyltransferase (GalT), from the Golgi. Analyses using chimeric proteins, in which the cytoplasmic regions of GalNAc-T2 and GalT were exchanged, revealed that the cytoplasmic region of GalNAc-T2 plays a crucial role in its TMF-dependent Golgi retention. These observations suggest critical roles for TMF in two Rab6-dependent retrograde transport processes: one from endosomes to the Golgi and the other from the Golgi to the ER.     

Rab cascades and tethering factors in the endomembrane system

Rab GTPases are key proteins that determine organelle identity and operate at the center of fusion reactions. Like Ras, they act as switches that are connected to a diverse network of tethering factors, exchange factors and GTPase activating proteins. Recent studies suggest that Rabs are linked to each other via their effectors, thus coordinating protein transport in the endomembrane system. Within this review, we will focus on selected examples that highlight these issues.     

Cellular functions of NSF: not just SNAPs and SNAREs

N-ethylmaleimide sensitive factor (NSF) is an ATPases associated with various cellular activities protein (AAA), broadly required for intracellular membrane fusion. NSF functions as a SNAP receptor (SNARE) chaperone which binds, through soluble NSF attachment proteins (SNAPs), to SNARE complexes and utilizes the energy of ATP hydrolysis to disassemble them thus facilitating SNARE recycling. While this is a major function of NSF, it does seem to interact with other proteins, such as the AMPA receptor subunit, GluR2, and beta2-AR and is thought to affect their trafficking patterns. New data suggest that NSF may be regulated by transient post-translational modifications such as phosphorylation and nitrosylation. These new aspects of NSF function as well as its role in SNARE complex dynamics will be discussed.