A role for regulated binding of p150 ^Glued to microtubule plus ends in organelle transport

A subset of microtubule-associated proteins, including cytoplasmic linker protein (CLIP)-170, dynactin, EB1, adenomatous polyposis coli, cytoplasmic dynein, CLASPs, and LIS-1, has been shown recently to target to the plus ends of microtubules. The mechanisms and functions of this binding specificity are not understood, although a role in encouraging microtubule elongation has been proposed. To extend previous work on the role of dynactin in organelle transport, we analyzed p150^Glued by live-cell imaging. Time-lapse analysis of p150^Glued revealed targeting to the plus ends of growing microtubules, requiring the NH₂-terminal cytoskeleton-associated protein–glycine rich domain, but not EB1 or CLIP-170. Effectors of protein kinase A modulated microtubule binding and suggested p150^Glued phosphorylation as a factor in plus-end binding specificity. Using a phosphosensitive monoclonal antibody, we mapped the site of p150^Glued phosphorylation to Ser-19. In vivo and in vitro analysis of phosphorylation site mutants revealed that p150^Glued phosphorylation mediates dynamic binding to microtubules. To address the function of dynamic binding, we imaged GFP-p150^Glued during the dynein-dependent transport of Golgi membranes. Live-cell analysis revealed a transient interaction between Golgi membranes and GFP-p150^Glued–labeled microtubules just prior to transport, implicating microtubules and dynactin in a search–capture mechanism for minus-end–directed organelles.