Annexin V: the membrane-binding protein with diverse functions

Annexin V is an eukaryotic protein from the annexin family which is able to reversibly bind to phospholipid membranes in a Ca2+-dependent manner. It possesses a complex mechanism of the membrane binding which includes the two-dimensional lattice formation from annexin V trimers and significant variation of the membrane structure. The precise functions of annexin V are largely unknown, however, its participation in the blood coagulation, membrane repair process and the Ca2+ channel activity is suggested. The usage of annexin V as a marker of phosphatidylserine-positive cells in in vitro and in vivo studies makes the understanding of the protein role in cellular processes critically important.

The current review is focused on the structure of annexin V and the mechanism and kinetics of its membrane binding. The lipid specificity and the multimerization process will be described. Finally, some of the proposed annexin V functions including inhibition of the blood coagulation and the Ca2+ transport activity will be discussed.

The annexin V structure. A. The view from the convex side. Magenta, N-terminal tail; blue, domain I; yellow, domain II; green, domain III; red, domain IV; orange, Ca2+ ions. In the center of annexin V the charged residues Asp280, Arg276, Asp92, Arg117, Glu112, Arg271 are represented. B. The view from the domain II. The convex and the concave sides are marked by black arrows. Ca2+-binding sites are located on the convex surface, N-terminal tail is on the concave side. Figures were created in VMD for the current review using the structure 1ANX [29] from PDB Data Bank. C. Annexin V from human (ANXA5_HUMAN) and from rat (ANXA5_RAT) sequences alignment. Residues that form the Ca2+-binding sites are highlighted in green and yellow for human and rat annexin V respectively. The alignment was done using the UniProt Align tool.
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#annexin A5#membrane interactions#calcium channel#inhibition of coagulation

Approaches to visualize microtubule dynamics in vitro

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#microtubules#light microscopy#atomic force microscopy#dynamic instability#tubulin#associated proteins

A minimal mathematical model of neutrophil pseudopodium formation during chemotaxis

The directed movement of neutrophils is provided by the rapid polymerization of actin with the formation of a protrusion growing forward. In our previous work we observed impaired neutrophil movement for patients with Wiskott-Aldrich syndrome (WAS) compared to healthy donors.

In this work, we set out to explain the impairment of neutrophil chemotaxis in patients by observation and computer modeling of the linear growth rates of the anterior pseudopodia. The neutrophil chemotaxis was observed by means of low-angle fluorescent microscopy in parallel-plate flow chambers. The computational model was constructed as a network-like 2D stochastic polymerization of actin guided by the proximity of cell membrane with branching governed by Arp2/3 and WASP proteins.

The observed linear velocity of neutrophil pseudopodium formation was 0.22 ± 0.04 μm/s for healthy donors and 0.23 ± 0.08 μm/s for WAS patients. The model described the velocity of the pseudopodium formation for healthy donors well. For the description of WAS patients data, a variation of branching velocity (governed by WASP) by an order of magnitude was applied, which did not significantly alter the linear protrusion growth velocity.

We conclude that the proposed mathematical model of neutrophil pseudopodium formation could describe the experimental data well, but the data on overall neutrophil movement could not be explained by the velocities of the pseudopodium growth.

Scheme of the computational model. (A) The scheme of stochastic events and species included in the model. A single F-actin filament is assumed to be straight and to be divided into segments. Each segment can be considered to be an actin monomer. New G-actin molecules can attach to and detach from the filament “barbed” end. It is assumed that the child filament begins to grow from the middle between two segments of F-actin at the angle of 70o.  If there is a branch growing from the F-actin segments, they are considered occupied and no branching can occur there. (B) The spatial restrictions on the filament growth and branching. The filaments can branch if the distance from the cell membrane is lesser than D. Filaments can grow if the distance from the cell membrane is lesser than H, where H > D.
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#cytoskeleton#neutrophils#actin#chemotaxis#Wiskott-Aldrich syndrome

STIM1-ORAI1 direct interaction cannot govern store-operated calcium entry (SOCE) in platelets

Store-operated calcium entry (SOCE) plays an important role in platelet function. It is generally assumed that the mechanism of SOCE relies on the direct interaction of STIM1 and ORAI1 proteins with specific STIM1:ORAI1 stoichiometry. However, in platelets, other pathways may take place. Here we aim to investigate the mechanisms of SOCE in platelets. We developed a lattice-based mathematical model that represented STIM1-ORAI1 interactions and applied it to both HEK cells, where SOCE mechanism is well established, and platelets. The model was able to describe STIM1-ORAI1 behavior in HEK cells successfully. We used the same parameters for protein interaction and applied them to platelets. As a result, we demonstrated that the number of STIM1 proteins on ER membrane could not assure the needed stoichiometry to proper SOCE in platelets.

(A) STIM1-ORAI1 interaction for HEK cells. Bound proteins are presented by brown, STIM1 in cluster – green, free proteins - red. (B) STIM1-ORAI1 puncta formation dependance on clustering probability. Dashed lines represent same process for D=0.01 〖μm〗^2/s (C) Typical system state at t=100ms for different clustering probabilities
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#platelets#mathematical modeling#store-operated calcium entry#platelet intracellular signaling