Disorder and structure in the Rab11 binding domain of Rab11 family interacting protein 2

Crystal structure of the Rab-binding domain of Rab11 family-interacting protein 2

The small GTPases Rab11, Rab14 and Rab25 regulate membrane trafficking through the recruitment of Rab11 family-interacting proteins (FIPs) to endocytic compartments. FIPs are multi-domain effector proteins that have a highly conserved Rab-binding domain (RBD) at their C-termini. Several structures of complexes of Rab11 with RBDs have previously been determined, including those of Rab11-FIP2 and Rab11-FIP3. In addition, the structures of the Rab14-FIP1 and Rab25-FIP2 complexes have been determined. All of the RBD structures contain a central parallel coiled coil in the RBD that binds to the switch 1 and switch 2 regions of the Rab. Here, the crystal structure of the uncomplexed RBD of FIP2 is presented at 2.3 Å resolution. The structure reveals antiparallel α-helices that associate through polar interactions. These include a remarkable stack of arginine residues within a four-helix bundle in the crystal lattice.

Influenza A virus ribonucleoproteins modulate host recycling by competing with Rab11 effectors

Influenza A virus assembly is an unclear process, whereby individual virion components form an infectious particle. The segmented nature of the influenza A genome imposes a problem to assembly because it requires packaging of eight distinct RNA particles (vRNPs). It also allows genome mixing from distinct parental strains, events associated with influenza pandemic outbreaks. It is important to public health to understand how segmented genomes assemble, a process that is dependent on the transport of components to assembly sites. Previously, it has been shown that vRNPs are carried by recycling endosome vesicles, resulting in a change of Rab11 distribution. Here, we describe that vRNP binding to recycling endosomes impairs recycling endosome function, by competing for Rab11 binding with family-interacting proteins, and that there is a causal relationship between Rab11 ability to recruit family-interacting proteins and Rab11 redistribution. This competition reduces recycling sorting at an unclear step, resulting in clustering of single- and double-membraned vesicles. These morphological changes in Rab11 membranes are indicative of alterations in protein and lipid homeostasis during infection. Vesicular clustering creates hotspots of the vRNPs that need to interact to form an infectious particle.

Rab11a Mediates Vascular Endothelial-Cadherin Recycling and Controls Endothelial Barrier Function

OBJECTIVE

Vascular endothelial (VE)-cadherin is the predominant component of endothelial adherens junctions essential for cell-cell adhesion and formation of the vascular barrier. Endocytic recycling is an important mechanism for maintaining the expression of cell surface membrane proteins. However, little is known about the molecular mechanism of VE-cadherin recycling and its role in maintenance of vascular integrity.

APPROACH AND RESULTS

Using calcium-switch assay, confocal imaging, cell surface biotinylation, and flow cytometry, we showed that VE-cadherin recycling required Ras-related proteins in brain (Rab)11a and Rab11 family-interacting protein 2. Yeast 2-hybrid assay and coimmunoprecipitation demonstrated that direct interaction of VE-cadherin with family-interacting protein 2 (at aa 453-484) formed a ternary complex with Rab11a in human endothelial cells. Silencing of Rab11a or Rab11 family-interacting protein 2 in endothelial cells prevented VE-cadherin recycling and VE-cadherin expression at endothelial plasma membrane. Furthermore, inactivation of Rab11a signaling blocked junctional reannealing after vascular inflammation. Selective knockdown of Rab11a in pulmonary microvessels markedly increased vascular leakage in mice challenged with lipopolysaccharide or polymicrobial sepsis.

CONCLUSIONS

Rab11a/Rab11 family-interacting protein 2-mediated VE-cadherin recycling is required for formation of adherens junctions and restoration of VE barrier integrity and hence a potential target for clinical intervention in inflammatory disease.

Rab11-FIP1C and Rab14 direct plasma membrane sorting and particle incorporation of the HIV-1 envelope glycoprotein complex

The incorporation of the envelope glycoprotein complex (Env) onto the developing particle is a crucial step in the HIV-1 lifecycle. The long cytoplasmic tail (CT) of Env is required for the incorporation of Env onto HIV particles in T cells and macrophages. Here we identify the Rab11a-FIP1C/RCP protein as an essential cofactor for HIV-1 Env incorporation onto particles in relevant human cells. Depletion of FIP1C reduced Env incorporation in a cytoplasmic tail-dependent manner, and was rescued by replenishment of FIP1C. FIP1C was redistributed out of the endosomal recycling complex to the plasma membrane by wild type Env protein but not by CT-truncated Env. Rab14 was required for HIV-1 Env incorporation, and FIP1C mutants incapable of binding Rab14 failed to rescue Env incorporation. Expression of FIP1C and Rab14 led to an enhancement of Env incorporation, indicating that these trafficking factors are normally limiting for CT-dependent Env incorporation onto particles. These findings support a model for HIV-1 Env incorporation in which specific targeting to the particle assembly microdomain on the plasma membrane is mediated by FIP1C and Rab14.

Enveloped viruses must develop strategies to ensure that a sufficient quantity of their receptor-binding envelope proteins are incorporated onto the surface of viruses as they form. The HIV envelope glycoprotein is specifically incorporated onto assembling virions in relevant cells such as T lymphocytes in a manner that requires its long cytoplasmic tail. The mechanism underlying this specific incorporation has remained unknown. Here, we identify a cellular trafficking pathway that is required for the incorporation of HIV envelope onto virions. A combination of the adaptor protein Rab11-FIP1C and Rab14 directs the envelope protein onto assembling virions, and loss of either of these host factors results in the production of virus particles lacking envelope. We also found that FIP1C was required for replication in T cell lines. This study identifies a trafficking complex required for HIV envelope incorporation and for the formation of infectious HIV particles.

Protein disorder in the centrosome correlates with complexity in cell types number

Here we study the properties and the evolution of proteins that constitute the Centrosome, the complex molecular assembly that regulates the division and differentiation of animal cells. We found that centrosomal proteins are predicted to be significantly enriched in disordered and coiled-coil regions, more phosphorylated and longer than control proteins of the same organism. Interestingly, the ratio of these properties in centrosomal and control proteins tends to increase with the number of cell-types. We reconstructed indels evolution, finding that indels significantly increase disorder in both centrosomal and control proteins, at a rate that is typically larger along branches associated with a large growth in cell-types number, and larger for centrosomal than for control proteins. Substitutions show a similar trend for coiled-coil, but they contribute less to the evolution of disorder. Our results suggest that the increase in cell-types number in animal evolution is correlated with the gain of disordered and coiled-coil regions in centrosomal proteins, establishing a connection between organism and molecular complexity. We argue that the structural plasticity conferred to the Centrosome by disordered regions and phosphorylation plays an important role in its mechanical properties and its regulation in space and time.

The Rab11a GTPase controls Toll-like receptor 4-induced activation of interferon regulatory factor-3 on phagosomes

Toll-like receptor 4 (TLR4) is indispensable for recognition of Gram-negative bacteria. We described a trafficking pathway for TLR4 from the endocytic recycling compartment (ERC) to E. coli phagosomes. We found a prominent colocalization between TLR4 and the small GTPase Rab11a in the ERC, and Rab11a was involved in the recruitment of TLR4 to phagosomes in a process requiring TLR4 signaling. Also, Toll-receptor-associated molecule (TRAM) and interferon regulatory factor-3 (IRF3) localized to E. coli phagosomes and internalization of E. coli was required for a robust interferon-β induction. Suppression of Rab11a reduced TLR4 in the ERC and on phagosomes leading to inhibition of the IRF3 signaling pathway induced by E. coli, whereas activation of the transcription factor NF-κB was unaffected. Moreover, Rab11a silencing reduced the amount of TRAM on phagosomes. Thus, Rab11a is an important regulator of TLR4 and TRAM transport to E. coli phagosomes thereby controlling IRF3 activation from this compartment.

Charge effects in the selection of NPF motifs by the EH domain of EHD1

The Eps15 homology (EH) domain is found in proteins associated with endocytosis and vesicle trafficking. EH domains bind to their target proteins through an asparagine-proline-phenylalanine (NPF) motif. We have measured the interaction energetics of the EH domain from EHD1 with peptides derived from two of its binding partners: Rabenosyn-5 (Ac-GPSLNPFDEED-NH(2)) and Rab11-Fip2 (Ac-YESTNPFTAK-NH(2)). Heteronuclear single quantum coherence (HSQC) spectroscopy shows that both peptides bind in the canonical binding pocket of EHD1 EH and induce identical structural changes, yet the affinity of the negatively charged Ac-GPSLNPFDEED-NH(2) (K(a) = 8 x 10(5) M(-1)) is tighter by 2 orders of magnitude. The thermodynamic profiles (DeltaG, DeltaH, DeltaS) were measured for both peptides as a function of temperature. The enthalpies of binding are essentially identical, and the difference in affinity is a consequence of the difference in entropic cost. Ac-GPSLNPFDEED-NH(2) binding is salt-dependent, demonstrating an electrostatic component to the interaction, whereas Ac-YESTNPFTAK-NH(2) binding is independent of salt. Successive replacement of acidic residues in Ac-GPSLNPFDEED-NH(2) with neutral residues showed that all are important. Lysine side chains in EHD1 EH create a region of strong positive surface potential near the NPF binding pocket. Contributions by lysine epsilon-amino groups to complex formation with Ac-GPSLNPFDEED-NH(2) was shown using direct-observe (15)N NMR spectroscopy. These experiments have enabled us to define a new extended interaction motif for EHD proteins, N-P-F-[DE]-[DE]-[DE], which we have used to predict new interaction partners and hence broaden the range of cellular activities involving the EHD proteins.

The dynamic Rab11-FIPs

The Rab11-FIPs (Rab11-family interacting proteins; also known as FIPs) constitute an evolutionarily conserved protein family that act as effector molecules for multiple Rab and Arf (ADP-ribosylation factor) GTPases. They were initially characterized by their ability to bind Rab11 subfamily members via a highly-conserved C-terminal RBD (Rab11-binding domain). Resolution of the crystal structure of Rab11 in complex with FIPs revealed that the RBD mediates homodimerization of the FIP molecules, creating two symmetrical interfaces for Rab11 binding and leading to the formation of a heterotetrameric complex between two FIP and two Rab11 molecules. The FIP proteins are encoded by five genes and alternative splicing has been reported. Based on primary structure, the FIPs were subcategorized into two classes: class I [Rip11, FIP2 and RCP (Rab-coupling protein)] and class II (FIP3 and FIP4). Recent studies have identified the FIPs as key players in the regulation of multiple distinct membrane trafficking events. In this mini-review, we summarize the Rab11-FIP field and discuss, at molecular and cellular levels, the recent findings on FIP function.