dictyNews

Electronic Edition

Volume 46, number 16

June 12, 2020



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Abstracts

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Copine A regulates the size and exocytosis of contractile vacuoles and 

postlysosomes in Dictyostelium



Elise M. Wight, Amber D. Ide, Cynthia K. Damer





FEBS Open Bio: published April 30th, 2020

https://doi.org/10.1002/2211-5463.12874 



Copines are a family of cytosolic proteins that associate with membranes 

in a calcium-dependent manner and are found in many eukaryotic organ- 

isms. Dictyostelium discoideum has six copine genes (cpnA-cpnF), and 

cells lacking cpnA (cpnA-) have defects in cytokinesis, chemotaxis, 

adhesion, and development. CpnA has also been shown to associate with 

the plasma membrane, contractile vacuoles (CV), and organelles of the 

endolysosomal pathway. Here, we use cpnA- cells to investigate the role 

of CpnA in CV function and endocytosis. When placed in water, cpnA-

cells made abnormally large CVs that took longer to expel. Visualization of 

CVs with the marker protein GFP-dajumin indicated that cpnA- cells had 

fewer CVs that sometimes refilled before complete emptying. In 

endocytosis assays, cpnA- cells took up small fluorescent beads by 

macropinocytosis at rates similar to parental cells. However, cpnA- cells 

reached a plateau sooner than parental cells and had less fluorescence at 

later time points. p80 anti- body labeling of postlysosomes (PL) indicated 

that there were fewer and smaller PLs in cpnA- cells. In dextran pulse-chase 

experiments, the number of PLs peaked earlier in cpnA- cells, and the PLs

 did not become as large and disappeared sooner as compared to parental 

 cells. PLs in cpnA- cells were also shown to have more actin coats, 

 suggesting CpnA may play a role in actin filament disassembly on PL 

 membranes. Overall, these results indicate that CpnA is involved in the 

 regulation of CV size and expulsion, and the maturation, size, and 

 exocytosis of PLs.





submitted by:  Amber Ide   [[log in to unmask]]

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Coordinated Ras and Rac activity shapes macropinocytic cups and enables 

phagocytosis of geometrically diverse bacteria   



Catherine M Buckley, Richard Potts, Aurelie Gueho, James H Vines, 

Christopher J Munn, Ben A Phillips, Berndt Gilsbach, Anton Nikolaev, 

Thierry Soldati, Andrew J Parnell, Arjan  Kortholt and Jason S King  





Current Biology 2020. 30, 1–15

https://doi.org/10.1016/j.cub.2020.05.049 



Engulfment of extracellular material by phagocytosis or macropinocytosis 

depends on the ability of cells to generate specialized cup-shaped 

protrusions. To effectively capture and internalize their targets, these cups 

are organized into a ring or ruffle of actin-driven protrusion encircling a non-

protrusive interior domain. These functional domains depend on the 

combined activities of multiple Ras and Rho family small GTPases, but how 

their activities are integrated and differentially regulated over space and time 

is unknown. Here, we show that the amoeba Dictyostelium discoideum 

coordinates Ras and Rac activity using the multidomain protein RGBARG 

(RCC1, RhoGEF, BAR, and RasGAP-containing protein). We find RGBARG 

uses a tripartite mechanism of Ras, Rac, and phospholipid interactions to 

localize at the protruding edge and interface with the interior of both 

macropinocytic and phagocytic cups. There, we propose RGBARG shapes 

the protrusion by expanding Rac activation at the rim while suppressing 

expansion of the active Ras interior domain. Consequently, cells lacking 

RGBARG form enlarged, flat interior domains unable to generate large 

macropinosomes. During phagocytosis, we find that disruption of RGBARG 

causes a geometry-specific defect in en- gulfing rod-shaped bacteria and 

ellipsoidal beads. This demonstrates the importance of coordinating small 

GTPase activities during engulfment of more complex shapes and thus the 

full physiological range of microbes, and how this is achieved in a model 

professional phagocyte.





submitted by:  Jason King   [[log in to unmask]]

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[End dictyNews, volume 46, number 16]