Actin and microtubules interact via MAPs

Circumstantial evidence rarely stands up in a court of law. In science, correlation has a little more credence, especially when it leads to a testable hypothesis. By 1978, people had been studying microtubules for a dozen years and nonmuscle actin filaments for about nine years. There were “plenty of examples in electron micrographs where microtubule and actin filaments were in the same place in the cell,” says Thomas Pollard (Yale University, New Haven, CT).

But, he notes, no one had investigated whether the polymer filaments actually interacted with each other and, if so, by what molecular connections. After seeing more provocative but still circumstantial images at a summer Woods Hole Marine Biological Laboratory course, Linda Griffith, then a graduate student at UCLA, asked Pollard if she could transfer to his Harvard laboratory to test the idea.

The two used a rather low-tech viscometer that measured a ball bearing's rate of fall through a capillary tube filled with actin filaments and microtubules (Griffith and Pollard, 1978). Pollard says the idea came from engineers at MIT who were using the falling ball method for other purposes. This simple method gave the team an easy biochemical assay, which caused less shearing of the filaments than traditional capillary viscometers.

When they added purified tubulin and actin to the tube and allowed filaments to polymerize, the viscosity was similar to the sum of the filaments' individual viscosities. But when microtubules were purified with their microtubule-associated proteins (MAPs), the viscosity of the mixture with actin jumped 120-fold. The experiments argued heavily for MAP-mediated cross-linking of microtubules and actin filaments.

The paper was one of the first to assign a molecular role to the MAPs beyond promoting microtubule polymerization. A few years later, Pollard's team showed that phosphorylation of MAPs inhibited the actin filament interaction (Selden and Pollard, 1983). Recently, microtubule–actin interactions have made headlines again, with advances in light microscopy revealing that the filaments interact at the leading edge of migrating live cells (Rodriguez et al., 2003).