Structural basis for VPS34 kinase activation by Rab1 and Rab5 on membranes.

The lipid phosphatidylinositol-3-phosphate (PI3P) is a regulator of two fundamental but distinct cellular processes, endocytosis and autophagy, so its generation needs to be under precise temporal and spatial control. PI3P is generated by two complexes that both contain the lipid kinase VPS34: complex II on endosomes (VPS34/VPS15/Beclin 1/UVRAG), and complex I on autophagosomes (VPS34/VPS15/Beclin 1/ATG14L). The endosomal GTPase Rab5 binds complex II, but the mechanism of VPS34 activation by Rab5 has remained elusive, and no GTPase is known to bind complex I. Here we show that Rab5a-GTP recruits endocytic complex II to membranes and activates it by binding between the VPS34 C2 and VPS15 WD40 domains. Electron cryotomography of complex II on Rab5a-decorated vesicles shows that the VPS34 kinase domain is released from inhibition by VPS15 and hovers over the lipid bilayer, poised for catalysis. We also show that the GTPase Rab1a, which is known to be involved in autophagy, recruits and activates the autophagy-specific complex I, but not complex II. Both Rabs bind to the same VPS34 interface but in a manner unique for each. These findings reveal how VPS34 complexes are activated on membranes by specific Rab GTPases and how they are recruited to unique cellular locations.

Conformational control of small GTPases by AMPylation.

Posttranslational modifications (PTMs) are important physiological means to regulate the activities and structures of central regulatory proteins in health and disease. Small GTPases have been recognized as important molecules that are targeted by PTMs during infections of mammalian cells by bacterial pathogens. The enzymes DrrA/SidM and AnkX from Legionella pneumophila AMPylate and phosphocholinate Rab1b during infection, respectively. Cdc42 is AMPylated by IbpA from Histophilus somni at tyrosine 32 or by VopS from Vibrio parahaemolyticus at threonine 35. These modifications take place in the important regulatory switch I or switch II regions of the GTPases. Since Rab1b and Cdc42 are central regulators of intracellular vesicular trafficking and of the actin cytoskeleton, their modifications by bacterial pathogens have a profound impact on the course of infection. Here, we addressed the biochemical and structural consequences of GTPase AMPylation and phosphocholination. By combining biochemical experiments and NMR analysis, we demonstrate that AMPylation can overrule the activity state of Rab1b that is commonly dictated by binding to guanosine diphosphate or guanosine triphosphate. Thus, PTMs may exert conformational control over small GTPases and may add another previously unrecognized layer of activity control to this important regulatory protein family.

Discovery of Rab1 binding sites using an ensemble of clustering methods.

Targeting non-native-ligand binding sites for potential investigative and therapeutic applications is an attractive strategy in proteins that share common native ligands, as in Rab1 protein. Rab1 is a subfamily member of Rab proteins, which are members of Ras GTPase superfamily. All Ras GTPase superfamily members bind to native ligands GTP and GDP, that switch on and off the proteins, respectively. Rab1 is physiologically essential for autophagy and transport between endoplasmic reticulum and Golgi apparatus. Pathologically, Rab1 is implicated in human cancers, a neurodegenerative disease, cardiomyopathy, and bacteria-caused infectious diseases. We have performed structural analyses on Rab1 protein using a unique ensemble of clustering methods, including multi-step principal component analysis, non-negative matrix factorization, and independent component analysis, to better identify representative Rab1 proteins than the application of a single clustering method alone does. We then used the identified representative Rab1 structures, resolved in multiple ligand states, to map their known and novel binding sites. We report here at least a novel binding site on Rab1, involving Rab1-specific residues that could be further explored for the rational design and development of investigative probes and/or therapeutic small molecules against the Rab1 protein. Proteins 2017; 85:859-871. © 2016 Wiley Periodicals, Inc.

Rab1 in cell signaling, cancer and other diseases.

The endoplasmic reticulum (ER) and Golgi membrane system have major roles in cell signaling and regulation of the biosynthesis/transport of proteins and lipids in response to environmental cues such as amino acid and cholesterol levels. Rab1 is the founding member of the Rab small GTPase family, which is known to mediate dynamic membrane trafficking between ER and Golgi. Growing evidence indicate that Rab1 proteins have important functions beyond their classical vesicular transport functions, including nutrient sensing and signaling, cell migration and presentation of cell-surface receptors. Moreover, deregulation of RAB1 expression has been linked to a myriad of human diseases such as cancer, cardiomyopathy and Parkinson's disease. Further investigating these new physiological and pathological functions of Rab1 should provide new opportunities for better understanding of the disease processes and may lead to more effective therapeutic interventions.

Adenylylation of Tyr77 stabilizes Rab1b GTPase in an active state: A molecular dynamics simulation analysis.

The pathogenic pathway of Legionella pneumophila exploits the intercellular vesicle transport system via the posttranslational attachment of adenosine monophosphate (AMP) to the Tyr77 sidechain of human Ras like GTPase Rab1b. The modification, termed adenylylation, is performed by the bacterial enzyme DrrA/SidM, however the effect on conformational properties of the molecular switch mechanism of Rab1b remained unresolved. In this study we find that the adenylylation of Tyr77 stabilizes the active Rab1b state by locking the switch in the active signaling conformation independent of bound GTP or GDP and that electrostatic interactions due to the additional negative charge in the switch region make significant contributions. The stacking interaction between adenine and Phe45 however, seems to have only minor influence on this stabilisation. The results may also have implications for the mechanistic understanding of conformational switching in other signaling proteins.

Structural basis for Rab1 de-AMPylation by the Legionella pneumophila effector SidD.

The covalent attachment of adenosine monophosphate (AMP) to proteins, a process called AMPylation (adenylylation), has recently emerged as a novel theme in microbial pathogenesis. Although several AMPylating enzymes have been characterized, the only known virulence protein with de-AMPylation activity is SidD from the human pathogen Legionella pneumophila. SidD de-AMPylates mammalian Rab1, a small GTPase involved in secretory vesicle transport, thereby targeting the host protein for inactivation. The molecular mechanisms underlying Rab1 recognition and de-AMPylation by SidD are unclear. Here, we report the crystal structure of the catalytic region of SidD at 1.6 Å resolution. The structure reveals a phosphatase-like fold with additional structural elements not present in generic PP2C-type phosphatases. The catalytic pocket contains a binuclear metal-binding site characteristic of hydrolytic metalloenzymes, with strong dependency on magnesium ions. Subsequent docking and molecular dynamics simulations between SidD and Rab1 revealed the interface contacts and the energetic contribution of key residues to the interaction. In conjunction with an extensive structure-based mutational analysis, we provide in vivo and in vitro evidence for a remarkable adaptation of SidD to its host cell target Rab1 which explains how this effector confers specificity to the reaction it catalyses.

Adenylylation, MS, and proteomics--Introducing a "new" modification to bottom-up proteomics.

Although the addition of a 5'-adenosine phosphodiester group to proteins, called adenylylation, has been known for decades, the possibility that adenylylation could be a molecular switch in cellular signaling pathways has emerged recently. The distinct mass shift upon adenylation of threonine or tyrosine residues renders it a good target for MS detection and identification; however, the fragmentation of adenylylated peptides derived from proteolytic digestion of adenylylated proteins has not yet been systematically investigated. Here, we demonstrate that adenylylated peptides show loss of parts of the adenosine monophosphate (AMP) upon different fragmentation techniques. As expected, causing the least fragmentation of the AMP group, electron transfer dissociation yields less complicated spectra. In contrast, CID and higher energy collision (HCD) fragmentation caused AMP to fragment, generating characteristic ions that could be utilized in the specific identification of adenylylated peptides. The characteristic ions and losses upon CID and higher energy collision fragmentation from the AMP group turned out to be highly dependent on which amino acid was adenylylated, with different reporter ions for adenylylated threonine and tyrosine. We also investigated how adenylylation is best incorporated into search engines, exemplified by Mascot and showed that it is possible to identify adenylylation by search engines.

Characterization of enzymes from Legionella pneumophila involved in reversible adenylylation of Rab1 protein.

After the pathogenic bacterium Legionella pneumophila is phagocytosed, it injects more than 250 different proteins into the cytoplasm of host cells to evade lysosomal digestion and to replicate inside the host cell. Among these secreted proteins is the protein DrrA/SidM, which has been shown to modify Rab1b, a main regulator of vesicular trafficking in eukaryotic cells, by transfer of adenosine monophosphate (AMP) to Tyr(77). In addition, Legionella provides the protein SidD that hydrolytically reverses the covalent modification, suggesting a tight spatial and temporal control of Rab1 function by Legionella during infection. Small angle x-ray scattering experiments of DrrA allowed us to validate a tentative complex model built by combining available crystallographic data. We have established the effects of adenylylation on Rab1 interactions and properties in a quantitative way. In addition, we have characterized the kinetics of DrrA-catalyzed adenylylation as well as SidD-catalyzed deadenylylation toward Rab1 and have determined the nucleotide specificities of both enzymes. This study enhances our knowledge of proteins subverting Rab1 function at the Legionella-containing vacuole.

Rab1 guanine nucleotide exchange factor SidM is a major phosphatidylinositol 4-phosphate-binding effector protein of Legionella pneumophila.

The causative agent of Legionnaires disease, Legionella pneumophila, forms a replicative vacuole in phagocytes by means of the intracellular multiplication/defective organelle trafficking (Icm/Dot) type IV secretion system and translocated effector proteins, some of which subvert host GTP and phosphoinositide (PI) metabolism. The Icm/Dot substrate SidC anchors to the membrane of Legionella-containing vacuoles (LCVs) by specifically binding to phosphatidylinositol 4-phosphate (PtdIns(4)P). Using a nonbiased screen for novel L. pneumophila PI-binding proteins, we identified the Rab1 guanine nucleotide exchange factor (GEF) SidM/DrrA as the predominant PtdIns(4)P-binding protein. Purified SidM specifically and directly bound to PtdIns(4)P, whereas the SidM-interacting Icm/Dot substrate LidA preferentially bound PtdIns(3)P but also PtdIns(4)P, and the L. pneumophila Arf1 GEF RalF did not bind to any PIs. The PtdIns(4)P-binding domain of SidM was mapped to the 12-kDa C-terminal sequence, termed "P4M" (PtdIns4P binding of SidM/DrrA). The isolated P4M domain is largely helical and displayed higher PtdIns(4)P binding activity in the context of the alpha-helical, monomeric full-length protein. SidM constructs containing P4M were translocated by Icm/Dot-proficient L. pneumophila and localized to the LCV membrane, indicating that SidM anchors to PtdIns(4)P on LCVs via its P4M domain. An L. pneumophila DeltasidM mutant strain displayed significantly higher amounts of SidC on LCVs, suggesting that SidM and SidC compete for limiting amounts of PtdIns(4)P on the vacuole. Finally, RNA interference revealed that PtdIns(4)P on LCVs is specifically formed by host PtdIns 4-kinase IIIbeta. Thus, L. pneumophila exploits PtdIns(4)P produced by PtdIns 4-kinase IIIbeta to anchor the effectors SidC and SidM to LCVs.

uDENN, DENN, and dDENN: indissociable domains in Rab and MAP kinases signaling pathways.

DENN domains are found in a variety of signaling proteins but their exact function remains undefined. Some of the DENN-containing proteins, such as rat Rab3GEP (Rab3 GDP/GTP exchange protein) or mouse Rab6IP1 (Rab6 interacting protein 1) interact with GTPases of the Rab family. Others, such as human MADD (MAP (Mitogen-activated protein) kinase activating protein containing death domain) and human ST5 (Suppressor of tumoreginicity 5) gene products are involved in regulation of MAPKs (Mitogen-activated protein kinases) signaling pathways. Using a combination of profile-based and bidimensional analyses, we show here that DENN domains are much larger than described to date in domain databases, always encircled on both sides by more divergent domains, that we called uDENN and dDENN. These however share conserved amino acids which could play a key role in the DENN functions.

A Rab1 homologue with a novel isoprenylation signal provides insight into the secretory pathway of Theileria parva.

As a first step in developing compartment-specific markers for protein trafficking within Theileria parva, we have isolated cDNAs encoding homologues of the small GTP binding proteins Rab1 and Rab4. The T. parva homologue of Rab1 (TpRab1), a protein which regulates vesicular transport between the endoplasmic reticulum and cis golgi in other organisms, was unusual in that it contained a unique 17 amino acid C-terminal extension. The C-terminal motif sequence KCT (XCX) contrasted with the CXC or XCC motifs which act as as signals for isoprenylation by geranylgeranyl in most Rab proteins, including all known Rab1 homologues, in containing only a single cysteine. [C14]mevalonic acid lactone and [H3]geranylgeranyl pyrophosphate were specifically incorporated into recombinant TpRab1 in vitro, demonstrating that the novel motif was functional for isoprenylation. Recombinant TpRab1 bound radiolabeled GTP, and this binding was inhibited by excess unlabeled GTP and GDP and also partially by ATP. The TpRab1 gene contained four short (34-67 bp) introns with a distinct pattern of occurrence within the protein sequence as compared to the introns of other lower eukaryote Rab1 genes. Immunofluorescence microscopy using antiserum specific for the novel C-terminal peptide in combination with labelling of cells using the nucleic acid-staining dye DAPI, indicated that TpRab1 was located in the vicinity of the schizont nucleus within the infected lymphocyte.