Heterotrimeric G Proteins and the Regulation of Microtubule Assembly

Abstract

Microtubules (MTs), a major component of cell cytoskeleton, exhibit diverse cellular functions including cell motility, intracellular transport, cell division, and differentiation.These functions of MTs are critically dependent on their ability to polymerize and depolymerize. Although a significant progress has been made in identifying cellular factors that regulate microtubule assembly and dynamics, the role of signal transducing molecules in this process is not well understood. It has been demonstrated that heterotrimeric G proteins, which are components of G protein-coupled receptor (GPCR) signaling pathway, interact with microtubules and play important roles in regulating assembly/dynamics of this cytoskeletal filament. While α subunit of G proteins (Gα) inhibits microtubule assembly and accelerates microtubule dynamics, Gβγ promotes tubulin polymerization. In this chapter, we review the current status of G-protein modulation of microtubules and cellular and physiological aspects of this regulation. Molecular, biochemical, and cellular methodologies that have been used to advance this field of research are discussed. Emphasis has been given on G-protein-microtubule interaction in neuronal differentiation as significant progress has been made in this field. The outcome from this research reflects the importance of GPCRs in transducing extracellular signals to regulate a variety of microtubule-associated cellular events.

Activation of β- and α2-adrenergic receptors stimulate tubulin polymerization and promote the association of Gβγ with microtubules in cultured NIH3T3 cells

“…To determine if receptor-mediated activation of G proteins alters tubulin polymerization (polymer-monomer equilibrium), NIH3T3 cells were treated with isoproterenol, an agonist for β-adrenergic receptors (β-AR). Binding of isoproterenol to the β-AR leads to the activation of Gs heterotrimer (Gsαβγ), followed by the dissociation of α and βγ subunits and subsequent activation of adenylyl cyclase [reviewed in 9, 10]. NIH3T3 cells were treated with 50 μM isoproterenol for 1 h, and MT and soluble tubulin (ST) fractions were prepared by extracting ST in a microtubule-stabilizing buffer (MS) as described previously [14, 15].…”

“…Subsequently, activated Gα changes its association with Gβγ in a manner that permits both subunits to participate in the regulation of intracellular effector molecules [6–8]. Although G proteins are likely to be membrane-bound when coupled to receptors, recent results from various laboratories suggest that G proteins associate with MTs, and regulate assembly/dynamics of MTs [reviewed in 9, 10]. It has been shown that α subunits of G proteins (Gsα, Gi1α and Gqα) inhibit MT assembly and promote MT dynamics in vitro [11].…”

Raman Spectroscopy Reveals Photobiomodulation-Induced α-Helix to β-Sheet Transition in Tubulins: Potential Implications for Alzheimer’s and Other Neurodegenerative Diseases

“…In the formation of MTs, αβ-tubulin binds guanosine triphosphate (GTP) at two different binding sites, one exchangeable (in β-tubulin) and one non-exchangeable (in αtubulin). Hydrolyzation of GTP at the exchangeable site allows tubulin assembly [10,22,23] into a mainly GDP-tubulin microtubule, with a final portion of GTP-bound tubulin known as the GTP cap. It is the presence of this cap that makes MT polymerization possible [23,24].…”

“…Hydrolyzation of GTP at the exchangeable site allows tubulin assembly [10,22,23] into a mainly GDP-tubulin microtubule, with a final portion of GTP-bound tubulin known as the GTP cap. It is the presence of this cap that makes MT polymerization possible [23,24]. When this piece of MTs is lost, the catastrophe phenomenon occurs, causing MTs to shrink instead of grow [24].…”