Microtubule-associated protein 1A: Heavy chain and light chain interactions

Abstract

Microtubule (Mt)-associated proteins (MAPs) have been shown to play a role in Mt stability in axons and dendrites, in determining neuronal shape and in regulating the balance between rigidity and plasticity in neuronal processes. MAP1A is the most abundant MAP in adult brains, and in neurons is localized in axons and dendrites. MAP1A is represented by a protein complex of one heavy chain (HC) and three light chains (LC1, LC2, LC3). Understanding MAP1A-Mt-LC interactions may give insight into the neuronal cytoskeleton, which is often involved in neuronal diseases. To determine which area of the MAP1A HC is essential in Mt binding, parts of the MAP1A HC were expressed in HeLa cells. Through fluorescence microscopy, we determined that MAP1A colocalized with Mts, but did not alter the Mt network. Also, an in vitro binding assay, where MAP1A HC fragments were added to taxol stabilized Mts, determined that the N-terminus of MAP1A is involved in Mt binding. To determine the effect of the LCs on Mts, they were also expressed in HeLa cells. By the same in vivo and in vitro experiments, all three LCs were shown to bind Mts. LC1 and LC2 also conferred to Mts a wavy distribution, especially in the perinuclear region and increased Mt stability as shown by increased resistance to nocodazole. However, Mts in cells transfected with LC3 were not altered and showed no increased Mt stability. Whenever both MAP1A HC and LC2 were expressed in HeLa cells, the altering effect of LC2 was reduced, suggesting that the MAP1A HC might regulate LC2 activities.