Membrane tubulin: Fact or fiction?

Tubulin is the ubiquitous protein that makes up the walls of the cytoskeletal elements known as microtubules. These 20 nm diameter cylindrical fibers are the spindle fibers for mitosis, provide the skeletal framework for cellular elongation, constitute the major structural and motile elements of cilia and flagella and probably play a number of other roles in eukaryote cells. In the electron microscope, they are never seen to attach or protrude directly into or on cellular membranes. It was therefore with much skepticism that cell biologists responded to recent claims purporting to show that tubulin is (or can be) a membrane protein. This skepticism comes from a bias based on the generally understood function of cytoskeletal proteins as representing the intracellular ‘bones’ of cells. The traditional thinking was that microtubules may attach to cellular membranes but only via a cross‐linking connecting protein. In recent years, however, an increasing number of workers have come to believe that tubulin can exist as a normal membrane protein possibly independent of its role in microtubule function.