An Essential Role for the Interaction Between Hyaluronan and Hyaluronan Binding Proteins During Joint Development

We studied the expression of hyaluronan binding proteins (HABPs) during the development of embryonic chick joints, using immunocytochemistry and biotinylated HA. The expression of actin capping proteins and of actin itself was also studied because the cytoskeleton is important in controlling HA-HABP interactions. Three cell surface HABPs were localized in the epiphyseal cartilage, articular fibrocartilage, and interzone that comprise the developing joint. Of these three HABPs, CD44 was associated with the articular fibrocartilages and interzone, whereas RHAMM and the IVd4 epitope were associated with all three tissues. Biotinylated HA was localized to interzone and articular fibrocartilages before cavity formation and within epiphyseal chondrocytes post cavitation. Actin filament bundles were observed at the developing joint line, as was the expression of the actin capping protein moesin. Manipulation of joint cavity development, using oligosaccharides of HA, disrupted joint formation and was associated with decreases in CD44 and actin filament expression as well as decreased hyaluronan synthetic capability. These results suggest that HA is actively bound by CD44 at the developing joint line and that HA-HABP interactions play a major role in the initial separation events occurring during joint formation.

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The structure of the actin filament uncapping complex mediated by twinfilin

Science Advances ◽

10.1126/sciadv.abd5271 ◽

2021 ◽

Vol 7 (5) ◽

pp. eabd5271

Author(s):

Dennis M. Mwangangi ◽

Edward Manser ◽

Robert C. Robinson

Keyword(s):

Actin Filament ◽

Protein Interactions ◽

Protein Complex ◽

Actin Filaments ◽

Actin Binding ◽

Capping Protein ◽

Binding Surface ◽

Actin Capping Protein ◽

Actin Capping

Uncapping of actin filaments is essential for driving polymerization and depolymerization dynamics from capping protein–associated filaments; however, the mechanisms of uncapping leading to rapid disassembly are unknown. Here, we elucidated the x-ray crystal structure of the actin/twinfilin/capping protein complex to address the mechanisms of twinfilin uncapping of actin filaments. The twinfilin/capping protein complex binds to two G-actin subunits in an orientation that resembles the actin filament barbed end. This suggests an unanticipated mechanism by which twinfilin disrupts the stable capping of actin filaments by inducing a G-actin conformation in the two terminal actin subunits. Furthermore, twinfilin disorders critical actin-capping protein interactions, which will assist in the dissociation of capping protein, and may promote filament uncapping through a second mechanism involving V-1 competition for an actin-binding surface on capping protein. The extensive interactions with capping protein indicate that the evolutionary conserved role of twinfilin is to uncap actin filaments.

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Structural insights into the regulation of actin capping protein by twinfilin C-terminal tail

Journal of Molecular Biology ◽

10.1016/j.jmb.2021.166891 ◽

2021 ◽

pp. 166891

Author(s):

Shuichi Takeda ◽

Ryotaro Koike ◽

Ikuko Fujiwara ◽

Akihiro Narita ◽

Makoto Miyata ◽

...

Keyword(s):

Capping Protein ◽

Actin Capping Protein ◽

Actin Capping ◽

Structural Insights

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Ca2+-dependent actin-binding phosphoprotein in Physarum polycephalum. Subunit b is a DNase I-binding and F-actin capping protein.

Journal of Biological Chemistry ◽

10.1016/s0021-9258(17)42976-0 ◽

1984 ◽

Vol 259 (8) ◽

pp. 5208-5213

Author(s):

H Maruta ◽

G Isenberg

Keyword(s):

Physarum Polycephalum ◽

Dnase I ◽

Actin Binding ◽

Capping Protein ◽

Actin Capping Protein ◽

Actin Capping ◽

Subunit B

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CapZ integrates several signaling pathways in response to mechanical stiffness

The Journal of General Physiology ◽

10.1085/jgp.201812199 ◽

2019 ◽

Vol 151 (5) ◽

pp. 660-669 ◽

Cited By ~ 3

Author(s):

Christopher Solís ◽

Brenda Russell

Keyword(s):

Mechanical Load ◽

Phosphorylation Site ◽

Tight Binding ◽

Neonatal Rat ◽

Actin Binding ◽

Mechanical Stimuli ◽

Muscle Adaptation ◽

Capping Protein ◽

Actin Capping Protein ◽

Actin Capping

Muscle adaptation is a response to physiological demand elicited by changes in mechanical load, hormones, or metabolic stress. Cytoskeletal remodeling processes in many cell types are thought to be primarily regulated by thin filament formation due to actin-binding accessory proteins, such as the actin-capping protein. Here, we hypothesize that in muscle, the actin-capping protein (named CapZ) integrates signaling by a variety of pathways, including phosphorylation and phosphatidylinositol 4,5-bisphosphate (PIP2) binding, to regulate muscle fiber growth in response to mechanical load. To test this hypothesis, we assess mechanotransduction signaling that regulates muscle growth using neonatal rat ventricular myocytes cultured on substrates with the stiffness of the healthy myocardium (10 kPa), fibrotic myocardium (100 kPa), or glass. We investigate how PIP2 signaling affects CapZ using the PIP2 sequestering agent neomycin and the effect of PKC-mediated CapZ phosphorylation using the PKC-activating drug phorbol 12-myristate 13-acetate (PMA). Molecular simulations suggest that close interactions between PIP2 and the β-tentacle of CapZ are modified by phosphorylation at T267. Fluorescence recovery after photobleaching (FRAP) demonstrates that the kinetic binding constant of CapZ to sarcomeric thin filaments in living muscle cells increases with stiffness or PMA treatment but is diminished by PIP2 reduction. Furthermore, CapZ with a deletion of the β-tentacle that lacks the phosphorylation site T267 shows increased FRAP kinetics with lack of sensitivity to PMA treatment or PIP2 reduction. Förster resonance energy transfer (FRET) probes the molecular interactions between PIP2 and CapZ, which are decreased by PIP2 availability or by the β-tentacle truncation. These data suggest that CapZ is bound to actin tightly in the idle, locked state, with little phosphorylation or PIP2 binding. However, this tight binding is loosened in growth states triggered by mechanical stimuli such as substrate stiffness, which may have relevance to fibrotic heart disease.

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