A CDC25 family protein phosphatase gates cargo recognition by the Vps26 retromer subunit

Receptor Recycling by Retromer

The highly conserved retromer complex controls the fate of hundreds of receptors that pass through the endolysosomal system and is a central regulatory node for diverse metabolic programs. More than 20 years ago, retromer was discovered as an essential regulator of endosome-to-Golgi transport in yeast; since then, significant progress has been made to characterize how metazoan retromer components assemble to enable its engagement with endosomal membranes, where it sorts cargo receptors from endosomes to the trans-Golgi network or plasma membrane through recognition of sorting motifs in their cytoplasmic tails. In this review, we examine retromer regulation by exploring its assembled structure with an emphasis on how a range of adaptor proteins shape the process of receptor trafficking. Specifically, we focus on how retromer is recruited to endosomes, selects cargoes, and generates tubulovesicular carriers that deliver cargoes to target membranes. We also examine how cells adapt to distinct metabolic states by coordinating retromer expression and function. We contrast similarities and differences between retromer and its related complexes: retriever and commander/CCC, as well as their interplay in receptor trafficking. We elucidate how loss of retromer regulation is central to the pathology of various neurogenerative and metabolic diseases, as well as microbial infections, and highlight both opportunities and cautions for therapeutics that target retromer. Finally, with a focus on understanding the mechanisms that govern retromer regulation, we outline new directions for the field moving forward.

MoMih1 is indispensable for asexual development, cell wall integrity, and pathogenicity of Magnaporthe oryzae

Asexual spore serves as essential inoculum of rice blast during the disease cycle, and differentiation of young conidium from conidiophore is intimately regulated by cell cycle. Mih1 encodes a dual-specificity phosphatase that involved in the G2/M transition of the mitotic cell cycle by regulating the Cdk1 activity in eukaryotes. Till now, the roles of Mih1 homologue, however, remain unclear in Magnaporthe oryzae. We here functionally characterized the Mih1 homologue MoMih1 in M. oryzae. MoMih1 is localized to both the cytoplasm and nucleus and can physically interact with the CDK protein MoCdc28 in vivo. Loss of MoMih1 led to delayed nucleus division and a high level of Tyr15 phosphorylation of MoCdc28. The MoMih1 mutants showed retarded mycelial growth with a defective polar growth, less fungal biomass, and shorter distance between diaphragms, compared with the KU80. Asexual reproduction altered in MoMih1 mutants, with both abnormal conidial morphogenesis and decreased conidiation. The MoMih1 mutants severely attenuated the virulence to host plants due to the impaired ability of penetration and biotrophic growth. The incapability of scavenging of host-derived reactive oxygen species, which was possibly ascribed to the severely decreased extracellular enzymes activities, were partially associated with deficiency of pathogenicity. Besides, the MoMih1 mutants displayed also improper localization of retromer protein MoVps26 and polarisome component MoSpa2, and defects of cell wall integrity (CWI), melanin pigmentation, chitin synthesis, and hydrophobicity. In conclusion, our results demonstrate that MoMih1 plays pleiotropic roles during fungal development and plant infection of M. oryzae.

Chitin Synthesis in Yeast: A Matter of Trafficking

Chitin synthesis has attracted scientific interest for decades as an essential part of fungal biology and for its potential as a target for antifungal therapies. While this interest remains, three decades ago, pioneering molecular studies on chitin synthesis regulation identified the major chitin synthase in yeast, Chs3, as an authentic paradigm in the field of the intracellular trafficking of integral membrane proteins. Over the years, researchers have shown how the intracellular trafficking of Chs3 recapitulates all the steps in the intracellular trafficking of integral membrane proteins, from their synthesis in the endoplasmic reticulum to their degradation in the vacuole. This trafficking includes specific mechanisms for sorting in the trans-Golgi network, regulated endocytosis, and endosomal recycling at different levels. This review summarizes the work carried out on chitin synthesis regulation, mostly focusing on Chs3 as a molecular model to study the mechanisms involved in the control of the intracellular trafficking of proteins.

Connecting the dots: combined control of endocytic recycling and degradation

Endocytosis is an essential process where proteins and lipids are internalised from the plasma membrane in membrane-bound carriers, such as clathrin-coated vesicles. Once internalised into the cell these vesicles fuse with the endocytic network where their contents are sorted towards degradation in the lysosome or recycling to their origin. Initially, it was thought that cargo recycling is a passive process, but in recent years the identification and characterisation of specialised recycling complexes has established a hitherto unthought-of level of complexity that actively opposes degradation. This review will summarise recent developments regarding the composition and regulation of the recycling machineries and their relationship with the degradative pathways of the endosome.

Architecture and mechanism of metazoan retromer:SNX3 tubular coat assembly

Retromer is a master regulator of cargo retrieval from endosomes, which is critical for many cellular processes including signaling, immunity, neuroprotection, and virus infection. The retromer core (VPS26/VPS29/VPS35) is present on cargo-transporting, tubular carriers along with a range of sorting nexins. Here, we elucidate the structural basis of membrane tubulation and coupled cargo recognition by metazoan and fungal retromer coats assembled with the non-Bin1/Amphiphysin/Rvs (BAR) sorting nexin SNX3 using cryo-electron tomography. The retromer core retains its arched, scaffolding structure but changes its mode of membrane recruitment when assembled with different SNX adaptors, allowing cargo recognition at subunit interfaces. Thus, membrane bending and cargo incorporation can be modulated to allow retromer to traffic cargoes along different cellular transport routes.

Phosphorylation of SNX27 by MAPK11/14 links cellular stress-signaling pathways with endocytic recycling

Endocytosed proteins can be delivered to lysosomes for degradation or recycled to either the trans-Golgi network or the plasma membrane. It remains poorly understood how the recycling versus degradation of cargoes is determined. Here, we show that multiple extracellular stimuli, including starvation, LPS, IL-6, and EGF treatment, can strongly inhibit endocytic recycling of multiple cargoes through the activation of MAPK11/14. The stress-induced kinases in turn directly phosphorylate SNX27, a key regulator of endocytic recycling, at serine 51 (Ser51). Phosphorylation of SNX27 at Ser51 alters the conformation of its cargo-binding pocket and decreases the interaction between SNX27 and cargo proteins, thereby inhibiting endocytic recycling. Our study indicates that endocytic recycling is highly dynamic and can crosstalk with cellular stress–signaling pathways. Suppression of endocytic recycling and enhancement of receptor lysosomal degradation serve as new mechanisms for cells to cope with stress and save energy.

Retrograde trafficking and plasma membrane recycling pathways of the budding yeast Saccharomyces cerevisiae

The endosomal system functions as a network of protein and lipid sorting stations that receives molecules from endocytic and secretory pathways and directs them to the lysosome for degradation, or exports them from the endosome via retrograde trafficking or plasma membrane recycling pathways. Retrograde trafficking pathways describe endosome-to-Golgi transport while plasma membrane recycling pathways describe trafficking routes that return endocytosed molecules to the plasma membrane. These pathways are crucial for lysosome biogenesis, nutrient acquisition and homeostasis and for the physiological functions of many types of specialized cells. Retrograde and recycling sorting machineries of eukaryotic cells were identified chiefly through genetic screens using the budding yeast Saccharomyces cerevisiae system and discovered to be highly conserved in structures and functions. In this review, we discuss advances regarding retrograde trafficking and recycling pathways, including new discoveries that challenge existing ideas about the organization of the endosomal system, as well as how these pathways intersect with cellular homeostasis pathways.

A bipartite sorting signal ensures specificity of retromer complex in membrane protein recycling

Retromer is an evolutionarily conserved protein complex, which sorts functionally diverse membrane proteins into recycling tubules/vesicles from the endosome. Many of the identified cargos possess a recycling signal sequence defined as ØX[L/M/V], where Ø is F/Y/W. However, this sequence is present in almost all proteins encoded in the genome. Also, several identified recycling sequences do not follow this rule. How then does retromer precisely select its cargos? Here, we reveal that an additional motif is also required for cargo retrieval. The two distinct motifs form a bipartite recycling signal recognized by the retromer subunits, Vps26 and Vps35. Strikingly, Vps26 utilizes different binding sites depending on the cargo, allowing retromer to recycle different membrane proteins. Thus, retromer interacts with cargos in a more complex manner than previously thought, which facilitates precise cargo recognition.

Dual-Specificity Phosphatase CDC25B Was Inhibited by Natural Product HB-21 Through Covalently Binding to the Active Site

Cysteine 473, within the active site of the enzyme, Cdc25B, is catalytically essential for substrate activation. The most well-reported inhibitors of Cdc25 phosphatases, especially quinone-type inhibitors, function by inducing irreversible oxidation at this active site of cysteine. Here, we identified a natural product, HB-21, having a sesquiterpene lactone skeleton that could irreversibly bind to cys473 through the formation of a covalent bond. This compound inhibited recombinant human Cdc25B phosphatase with an IC₅₀ value of 24.25 μM. Molecular modeling predicted that HB-21 not only covalently binds to cys473 of Cdc25B but also forms six hydrogen bonds with residues at the active site. Moreover, HB-21 can dephosphorylate cyclin-dependent kinase (CDK1), the natural substrate of Cdc25b, and inhibit cell cycle progression. In summary, HB-21 is a new type of Cdc25B inhibitor with a novel molecular mechanism.

To degrade or not to degrade: mechanisms and significance of endocytic recycling

Newly endocytosed integral cell surface proteins are typically either directed for degradation or subjected to recycling back to the plasma membrane. The sorting of integral cell surface proteins, including signalling receptors, nutrient transporters, ion channels, adhesion molecules and polarity markers, within the endolysosomal network for recycling is increasingly recognized as an essential feature in regulating the complexities of physiology at the cell, tissue and organism levels. Historically, endocytic recycling has been regarded as a relatively passive process, where the majority of internalized integral proteins are recycled via a nonspecific sequence-independent 'bulk membrane flow' pathway. Recent work has increasingly challenged this view. The discovery of sequence-specific sorting motifs and the identification of cargo adaptors and associated coat complexes have begun to uncover the highly orchestrated nature of endosomal cargo recycling, thereby providing new insight into the function and (patho)physiology of this process.

Updated Insight into the Physiological and Pathological Roles of the Retromer Complex

Retromer complexes mediate protein trafficking from the endosomes to the trans-Golgi network (TGN) or through direct recycling to the plasma membrane. In yeast, they consist of a conserved trimer of the cargo selective complex (CSC), Vps26-Vps35-Vps29 and a dimer of sorting nexins (SNXs), Vps5-Vps17. In mammals, the CSC interacts with different kinds of SNX proteins in addition to the mammalian homologues of Vps5 and Vps17, which further diversifies retromer functions. The retromer complex plays important roles in many cellular processes including restriction of invading pathogens. In this review, we summarize some recent developments in our understanding of the physiological and pathological functions of the retromer complex.