Tubulin is an Amphiphilic Protein Whose Interaction With Membranes is Regulated by its Charged Carboxy-Terminal Tails

Abstract

Tubulin is an acidic heterodimeric protein whose negative charges are significantly (∼40% of total negative charge) located on the 10-15 residue long, glutamic acid-rich, unstructured, carboxy-terminal tails (CTT) found on both α- and β-subunits (though not on the monomeric γ-tubulin). Unsurprisingly, tubulin is a quite water-soluble protein. However, it has been consistently reported to be a component of highly purified membranes, including plasma membranes, intracellular membranes such as Golgi and mitochondrial outer membranes, and vesicle membranes as in clathrin-coated endocytic vesicles. In these preparations, tubulin is non-microtubular and is tightly associated with the membrane, often requiring detergent for solubilization. Tubulin has also been shown to associate tightly with liposomes make of purified lipid only, including neutral lipid. The exact mechanism of tubulin-membrane association has not been defined, nor has it been shown that there is only one mechanism. Tubulin could dock with lipid-embedded proteins, for example. We have shown that tubulin binds to VDAC in the mitochondrial outer membrane, with functional consequences for mitochondrial function, and this binding requires and is mediated by the CTT. Thus the CTT can enhance membrane binding of tubulin. This cannot be the mechanism for liposome interaction, since there is no protein present other than tubulin. We show by charge-shift electrophoresis and non-ionic detergent extraction that (a) tubulin behaves as an amphiphilic protein, and (b) the CTT can regulate interaction with amphipathic molecules.