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#centrosome

4 public questions tagged with this topic.

Cytoskeleton

The γ-tubulin ring complex (γ-TuRC) functions to:

De novo microtubule formation faces thermodynamic barrier because tubulin dimers inefficiently oligomerize spontaneously requiring template. Gamma tubulin ring complex overcomes this kinetic hurdle by acting as structural template mimicking microtubule end geometry precisely. Complex forms lock washer shape about 30 nm diameter composed of gamma tubulin plus grip motif proteins GCP2 through GCP6 arranged with 13 gamma tubulin copies reproducing geometry of 13 protofilament microtubule plus end. Recruitment to centrosomes and spindle microtubules via pericentrin NEDD1 positions complex with gamma tubulins exposed outward. Alpha beta dimers longitudinally add onto gamma tubulins with alpha tubulin contacting gamma establishing polarity with minus end capped and stable. Functionally it nucleates new polymerization blocking minus end depolymerization and defining protofilament number. It does not hydrolyze GTP for regulation nor cap plus ends nor sever microtubules like katanin. Thus nucleation function essential for spindle and interphase array formation. Additional regulatory inputs including phosphorylation, small GTPases, and cargo adaptors fine tune filament assembly stability and motor activity matching cellular demands during division, migration, and mechanical stress responses efficiently.

Ref: Kollman et al., Nature 2010 – Gamma TuRC nucleates microtubule formation at minus end template mechanism.

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Cytoskeleton

Which structure serves as the nucleation site for microtubule polymerization in animal cells?

Cytoplasmic microtubule arrays need defined origin to orient trafficking and establish cell polarity and division symmetry. In animal cells primary microtubule organizing center is centrosome positioned near nucleus serving as focal nucleation point and signaling hub. Ultrastructurally centrosome comprises pair of cylindrical centrioles surrounded by amorphous pericentriolar material enriched in coiled coil proteins pericentrin Cep192 and gamma tubulin ring complexes gamma TuRC. Centrioles provide 9 fold symmetry scaffold stabilized by polyglutamylated triplets while PCM concentrates nucleation templates several hundred fold. Gamma TuRC anchored within PCM caps microtubule minus ends while allowing plus ends to grow outward generating radial aster exploring cell periphery. This organization directs kinesin plus end transport outward and dynein minus end inward positioning Golgi near centrosome and nucleus central. Golgi and basal bodies nucleate subsets but dominant interphase and mitotic aster originates from centrosome. Loss impairs polarity signaling and leads to supernumerary spindle formation. Additional regulatory inputs including phosphorylation, small GTPases, and cargo adaptors fine tune filament assembly stability and motor activity matching cellular demands during division, migration, and mechanical stress responses efficiently.

Ref: Doxsey, Nat Rev Mol Cell Biol 2001: Centrosome as nucleation site for microtubule polymerization organization.

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Cell Cycle Regulation

What is the primary function of the γ-TuRC nucleating structure in the centrosome?

Spontaneous microtubule assembly from alpha-beta tubulin heterodimers is kinetically unfavorable due to nucleation barrier requiring oligomeric seed. Centrosomes overcome this via gamma-tubulin ring complex, dominant template in animal cells. Gamma-TuRC assembles as 14 nm wide lock-washer containing about 14 gamma-tubulin molecules positioned with GCP2-6 proteins, namely GCP2, GCP3 forming gamma-TuSC repeats plus GCP4, GCP5, GCP6, plus actin and MZT1/2 small proteins. Structure mirrors 13-protofilament microtubule lattice with minus end capped. Localized within pericentriolar material through adaptors pericentrin, CDK5RAP2, CEP192, gamma-TuRC provides lateral contacts stabilizing nascent tubulin dimers, lowering critical concentration for polymerization and defining polarity with plus ends outward. Nucleation capacity expands dramatically at G2/M when CDK1, Plk1 and Aurora A mediated phosphorylation increases PCM recruitment. During mitosis, several hundred complexes per centrosome generate astral and spindle microtubules. Branching nucleation via augmin complex using existing microtubules further amplifies spindle density. Defective gamma-TuRC function leads to weak asters, monopolar spindles and failure of chromosome congression. Additional feedback loops involving polo-like kinases, phosphatases and SCF-mediated degradation reinforce irreversibility and protect against premature progression that would compromise genome integrity and viability.

Ref: Kollman et al., Gamma-TuRC Structure and Microtubule Nucleation, Nat Rev Mol Cell Biol 2011; Alberts et al., Chapter 16, Microtubule Organizing Center.

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