MAP2‐mediated binding of chromaffin granules to microtubules

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Abstract
We have examined the interaction of chromaffin granules from bovine adrenal modulla with microtubules. Chromaffin granules were mixed with microtubules made of phosphocellulose‐purified tubulin, and pelleted through a 1.6 M sucrose cushion at 12000 × g for 10 min. Both components (granules and microtubules) were pelleted when added together but not separately. This result indicates that granules form a heavy complex with the microtubules. Such a complex was visualized by an electron microscopy of the granule/microtubule mixture. Treatment of the granules with trypsin abolished their ability to interact with the microtubules. The binding of the granules to the microtubules: (i) was not sensitive to ATP: and (ii) was completely inhibited by the cleavage of C‐terminal peptides of α‐ and β‐subunits of tubulin with subtilisin. These relationships suggest that the granule binding is mediated by one of the structural microtubule‐associated proteins rather than by microtubule‐dependent translocators. For identification of protein(s) mediating the binding, the granules were solubilized with Triton X‐100, soluble proteins were mixed with the microtubules, and microtubules with bound proteins were pelleted through a glycerol cushion. At least one granule protein interacting with the microtubules was found in the pellet. This protein was identified as MAP2 according to its electrophoretic mobility and reactivity with a MAP2 antibody. Affinity chromatography of solubilized proteins on a column containing taxol‐stabilized microtubules also revealed MAP2 as a protein of chromaffin granules interacting with the microtubules.