Phosphoproteomic insights into processes influenced by the kinase-like protein DIA1/C3orf58

Allnighter pseudokinase-mediated feedback links proteostasis and sleep in Drosophila

In nervous systems, retrograde signals are key for organizing circuit activity and maintaining neuronal homeostasis. We identify the conserved Allnighter (Aln) pseudokinase as a cell non-autonomous regulator of proteostasis responses necessary for normal sleep and structural plasticity of Drosophila photoreceptors. In aln mutants exposed to extended ambient light, proteostasis is dysregulated and photoreceptors develop striking, but reversible, dysmorphology. The aln gene is widely expressed in different neurons, but not photoreceptors. However, secreted Aln protein is retrogradely endocytosed by photoreceptors. Inhibition of photoreceptor synaptic release reduces Aln levels in lamina neurons, consistent with secreted Aln acting in a feedback loop. In addition, aln mutants exhibit reduced night time sleep, providing a molecular link between dysregulated proteostasis and sleep, two characteristics of ageing and neurodegenerative diseases.

Regulation of secretory pathway kinase or kinase-like proteins in human cancers

Secretory pathway kinase or kinase-like proteins (SPKKPs) are effective in the lumen of the endoplasmic reticulum (ER), Golgi apparatus (GA), and extracellular space. These proteins are involved in secretory signaling pathways and are distinctive from typical protein kinases. Various reports have shown that SPKKPs regulate the tumorigenesis and progression of human cancer via the phosphorylation of various substrates, which is essential in physiological and pathological processes. Emerging evidence has revealed that the expression of SPKKPs in human cancers is regulated by multiple factors. This review summarizes the current understanding of the contribution of SPKKPs in tumorigenesis and the progression of immunity. With the epidemic trend of immunotherapy, targeting SPKKPs may be a novel approach to anticancer therapy. This study briefly discusses the recent advances regarding SPKKPs.

High-resolution structure of a fish aquaporin reveals a novel extracellular fold

The structural and functional characterization of a fish AQP reveals a water-specific AQP with unique structural features that may have implications for channel gating in response to osmotic changes.

Aquaporins are protein channels embedded in the lipid bilayer in cells from all organisms on earth that are crucial for water homeostasis. In fish, aquaporins are believed to be important for osmoregulation; however, the molecular mechanism behind this is poorly understood. Here, we present the first structural and functional characterization of a fish aquaporin; cpAQP1aa from the fresh water fish climbing perch (Anabas testudineus), a species that is of high osmoregulatory interest because of its ability to spend time in seawater and on land. These studies show that cpAQP1aa is a water-specific aquaporin with a unique fold on the extracellular side that results in a constriction region. Functional analysis combined with molecular dynamic simulations suggests that phosphorylation at two sites causes structural perturbations in this region that may have implications for channel gating from the extracellular side.

mTOR controls endoplasmic reticulum-Golgi apparatus trafficking of VSVg in specific cell types

BACKGROUND

Mammalian/mechanistic target of rapamycin (mTOR) complexes are essential for cell proliferation, growth, differentiation, and survival. mTORC1 hyperactivation occurs in the tuberous sclerosis complex (TSC). mTORC1 localizes to the surface of lysosomes, where Rheb activates it. However, mTOR was also found on the endoplasmic reticulum (ER) and Golgi apparatus (GA). Recent studies showed that the same inputs regulate ER-to-GA cargo transport and mTORC1 (e.g., the level of amino acids or energy status of the cell). Nonetheless, it remains unknown whether mTOR contributes to the regulation of cargo passage through the secretory pathway.

METHODS

The retention using selective hooks (RUSH) approach was used to image movement of model cargo (VSVg) between the ER and GA in various cell lines in which mTOR complexes were inhibited. We also investigated VSVg trafficking in TSC patient fibroblasts.

RESULTS

We found that mTOR inhibition led to the overall enhancement of VSVg transport through the secretory pathway in PC12 cells and primary human fibroblasts. Also, in TSC1-deficient cells, VSVg transport was enhanced.

CONCLUSIONS

Altogether, these data indicate the involvement of mTOR in the regulation of ER-to-GA cargo transport and suggest that impairments in exocytosis may be an additional cellular process that is disturbed in TSC.

The ABCs of the atypical Fam20 secretory pathway kinases

The study of extracellular phosphorylation was initiated in late 19th century when the secreted milk protein, casein, and egg-yolk protein, phosvitin, were shown to be phosphorylated. However, it took more than a century to identify Fam20C, which phosphorylates both casein and phosvitin under physiological conditions. This kinase, along with its family members Fam20A and Fam20B, defined a new family with altered amino acid sequences highly atypical from the canonical 540 kinases comprising the kinome. Fam20B is a glycan kinase that phosphorylates xylose residues and triggers peptidoglycan biosynthesis, a role conserved from sponges to human. The protein kinase, Fam20C, conserved from nematodes to humans, phosphorylates well over 100 substrates in the secretory pathway with overall functions postulated to encompass endoplasmic reticulum homeostasis, nutrition, cardiac function, coagulation, and biomineralization. The preferred phosphorylation motif of Fam20C is SxE/pS, and structural studies revealed that related member Fam20A allosterically activates Fam20C by forming a heterodimeric/tetrameric complex. Fam20A, a pseudokinase, is observed only in vertebrates. Loss-of-function genetic alterations in the Fam20 family lead to human diseases such as amelogenesis imperfecta, nephrocalcinosis, lethal and nonlethal forms of Raine syndrome with major skeletal defects, and altered phosphate homeostasis. Together, these three members of the Fam20 family modulate a diverse network of secretory pathway components playing crucial roles in health and disease. The overarching theme of this review is to highlight the progress that has been made in the emerging field of extracellular phosphorylation and the key roles secretory pathway kinases play in an ever-expanding number of cellular processes.

DIPK2A promotes STX17- and VAMP7-mediated autophagosome-lysosome fusion by binding to VAMP7B

Autophagosome and lysosome fusion is an important macroautophagy/autophagy process for cargo degradation, and SNARE proteins, including STX17, SNAP29, VAMP7 and VAMP8, are key players in this process. However, the manner in which this process is precisely regulated is poorly understood. Here, we show that VAMP7B, a SNARE domain-disrupted isoform of R-SNARE protein VAMP7, competes with SNARE domain functional isoform VAMP7A to bind to STX17 and inhibits autophagosome-lysosome fusion. Moreover, we show that DIPK2A, a late endosome- and lysosome-localized protein, binds to VAMP7B, which inhibits the interaction of VAMP7B with STX17 and enhances the binding of STX17 to VAMP7A, thus enhancing autophagosome-lysosome fusion. Furthermore, DIPK2A participates in autophagic degradation of mitochondria proteins and alleviates apoptosis. Thus, we reveal a new aspect of autophagosome-lysosome fusion in which different isoforms of VAMP7 compete with STX17 and their regulation by DIPK2A.Abbreviations: DIPK2A: divergent protein kinase domain 2A; EEA1: early endosome antigen 1; GOLGA2: golgin A2; LAMP1: lysosomal associated membrane protein 1; MAP1LC3B/LC3: microtubule associated protein 1 light chain 3 beta; MFN2: mitofusin 2; MT-CO2: mitochondrially encoded cytochrome c oxidase II; PARP1: poly(ADP-ribose) polymerase 1; PRKN: parkin RBR E3 ubiquitin protein ligase; RAB5A: RAB5A, member RAS oncogene family; RAB7A: RAB7A, member RAS oncogene family; REEP: receptor accessory protein; RTN4: reticulon 4; SNARE: SNAP receptor; SQSTM1/p62: sequestosome 1; STX17: syntaxin 17; TOMM20: translocase of outer mitochondrial membrane 20; VAMP7: vesicle associated membrane protein 7; VAMP8: vesicle associated membrane protein 8.

Thinking outside of the cell: Secreted protein kinases in bacteria, parasites, and mammals

Previous decades have seen an explosion in our understanding of protein kinase function in human health and disease. Hundreds of unique kinase structures have been solved, allowing us to create generalized rules for catalysis, assign roles of communities within the catalytic core, and develop specific drugs for targeting various pathways. Although our understanding of intracellular kinases has developed at a fast rate, our exploration into extracellular kinases has just begun. In this review, we will cover the secreted protein kinase families found in humans, bacteria, and parasites. © 2019 IUBMB Life, 71(6):749-759, 2019.