MST4 kinase phosphorylates ACAP4 protein to orchestrate apical membrane remodeling during gastric acid secretion

Digestion in the stomach depends on acidification of the lumen. Histamine-elicited acid secretion is triggered by activation of the PKA cascade, which ultimately results in the insertion of gastric H,K-ATPases into the apical plasma membranes of parietal cells. Our recent study revealed the functional role of PKA-MST4-ezrin signaling axis in histamine-elicited acid secretion. However, it remains uncharacterized how the PKA-MST4-ezrin signaling axis operates the insertion of H,K-ATPases into the apical plasma membranes of gastric parietal cells. Here we show that MST4 phosphorylates ACAP4, an ARF6 GTPase-activating protein, at Thr⁵⁴⁵ Histamine stimulation activates MST4 and promotes MST4 interaction with ACAP4. ACAP4 physically interacts with MST4 and is a cognate substrate of MST4 during parietal cell activation. The phosphorylation site of ACAP4 by MST4 was mapped to Thr⁵⁴⁵ by mass spectrometric analyses. Importantly, phosphorylation of Thr⁵⁴⁵ is essential for acid secretion in parietal cells because either suppression of ACAP4 or overexpression of non-phosphorylatable ACAP4 prevents the apical membrane reorganization and proton pump translocation elicited by histamine stimulation. In addition, persistent overexpression of MST4 phosphorylation-deficient ACAP4 results in inhibition of gastric acid secretion and blockage of tubulovesicle fusion to the apical membranes. Significantly, phosphorylation of Thr⁵⁴⁵ enables ACAP4 to interact with ezrin. Given the location of Thr⁵⁴⁵ between the GTPase-activating protein domain and the first ankyrin repeat, we reason that MST4 phosphorylation elicits a conformational change that enables ezrin-ACAP4 interaction. Taken together, these results define a novel molecular mechanism linking the PKA-MST4-ACAP4 signaling cascade to polarized acid secretion in gastric parietal cells.

Mitotic Protein CSPP1 Interacts with CENP-H Protein to Coordinate Accurate Chromosome Oscillation in Mitosis

Mitotic chromosome segregation is orchestrated by the dynamic interaction of spindle microtubules with the kinetochores. During chromosome alignment, kinetochore-bound microtubules undergo dynamic cycles between growth and shrinkage, leading to an oscillatory movement of chromosomes along the spindle axis. Although kinetochore protein CENP-H serves as a molecular control of kinetochore-microtubule dynamics, the mechanistic link between CENP-H and kinetochore microtubules (kMT) has remained less characterized. Here, we show that CSPP1 is a kinetochore protein essential for accurate chromosome movements in mitosis. CSPP1 binds to CENP-H in vitro and in vivo. Suppression of CSPP1 perturbs proper mitotic progression and compromises the satisfaction of spindle assembly checkpoint. In addition, chromosome oscillation is greatly attenuated in CSPP1-depleted cells, similar to what was observed in the CENP-H-depleted cells. Importantly, CSPP1 depletion enhances velocity of kinetochore movement, and overexpression of CSPP1 decreases the speed, suggesting that CSPP1 promotes kMT stability during cell division. Specific perturbation of CENP-H/CSPP1 interaction using a membrane-permeable competing peptide resulted in a transient mitotic arrest and chromosome segregation defect. Based on these findings, we propose that CSPP1 cooperates with CENP-H on kinetochores to serve as a novel regulator of kMT dynamics for accurate chromosome segregation.

Further New Metabolites From Lichens

The new depsidones didechlorolecideoidin (1) [methyl 3,8-dihydroxy-1,6-dimethyl-11-oxo-11H-dibenzo[ b,e ][1,4]dioxepin-7-carboxylate] and 4-dechlorogangaleoidin (2) [methyl 2-chloro-3-hydroxy-8-methoxy-1,6-dimethyl-11-oxo-11H-dibenzo[ b,e ][1,4]dioxepin-7-carboxylate] have been detected in the lichens Lecanora argentata and L. californica respectively, and the structure of these metabolites has been established by chromatographic comparisons with synthetic material. The compound (1) has also been shown to occur in the lichen Tylothallia pahiensis. In addition the dibenzofuran, furfuraceic acid [3,7-dihydroxy-1,9-di(2′-oxoheptyl)dibenzofuran-2-carboxylic acid] (9) has been isolated from the lichen Phyllopsora furfuracea, and the structure established by spectroscopic methods.

New Chlorine-Containing Depsidones From the Lichen Phyllopsora corallina var. ochroxantha

The depsidones phyllopsorin (2-chloro-4,6-diformyl-3-hydroxy-8-methoxy- 1,9-dimethyl-11H-dibenzo[ b,e ][1,4]dioxepin-11-one (3), chlorophyllopsorin (2,7-dichloro-4,6-diformyl-3-hydroxy-8-methoxy-1,9- dimethyl-11H-dibenzo[ b,e ][1,4]dioxepin-11-one) (4), methyl 2,7- dichloro-psoromate (methyl 2,7-dichloro-4-formyl-3-hydroxy-8-methoxy- 1,9-dimethyl-11-oxo-11H-dibenzo[ b,e ][1,4]dioxepin-6-carboxylate) (5) and methyl 2,7-dichloronorpsoromate (methyl 2,7-dichloro-4-formyl-3,8- dihydroxy-1,9-dimethyl-11-oxo-11H-dibenzo[ b,e ][1,4]dioxepin-6-carb- oxylate ) (6) have been isolated from the lichen Phyllopsora corallina var. ochroxantha. The structures of these metabolites have been established from the spectroscopic properties, degradative studies and by partial synthesis.

The intermediate filament cytoskeleton of macrophages

This study characterizes two- and three-dimensional ultrastructure and surface topography of polymerized networks of intermediate filaments (IF) isolated from mouse peritoneal macrophages. Isolated IF bound to monoclonal anti-IF antibodies in enzyme-linked immunosorbent assays. Immunogold labeling of IF with specific antibodies revealed that epitopes are distributed along filaments particularly at junctions where filaments interconnect. Networks of IF, viewed by scanning electron microscopy, organized as ropelike groups of interconnecting filaments which swirl and encircle each other to form three-dimensional lattices containing ellipsoidal-, circular-, and vacuole-shaped cavities. Cavity diameters were similar in size to organelles and vacuoles; diameters were grouped as small (12-288 nm), medium (0.3-1.7 micron), and large (2-3 micron). The walls of the cavities appeared as beaded structures with alternating globular and linear regions. Linear regions were 14 nm. Repeat distances taken from the central axis of globular regions were 23-27 nm. The lattice organization of IF observed in vitro was similar to images seen in vivo in Triton-insoluble cytoskeletons immunofluorescently labeled with specific antibodies. In whole cells processed for TEM, swirling bundles of IF were found encircling membranous vacuoles. Based on the lattice architecture of IF, cavity dimensions, and IF location, we postulate that intermediate filaments may function in the mechanical and spatial distribution of vacuoles in the cell cytoplasm.