The 72‐kDa Microtubule‐Associated Protein from Porcine Brain

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Abstract
A microtubule‐associated protein (MAP) with a molecular mass of 72‐kDa that was purified from porcine brain by using its property of heat stability in a low pH buffer was characterized. Low‐angle rotary shadowing revealed that the 72‐kDa protein was a rodlike protein ∼55–75 nm long. The 72‐kDa protein bound to microtubules polymerized from phosphocellulose column‐purified tubulin (PC‐tubulin) with taxol and promoted the polymerization of PC‐tubulin in the absence of taxol. Microtubules polymerized by the 72‐kDa protein showed a tendency to form bundles of several microtubules. Quick‐freeze, deep‐etch electron microscopy revealed that the 72‐kDa protein formed short crossbridges between microtubules. We performed peptide mapping to analyze the relationship of the 72‐kDa protein to other heat‐stable MAPs, and the results showed some resemblance of the 72‐kDa protein to MAP2. Cross‐reactivity with a monoclonal anti‐MAP2 antibody further suggested that the 72‐kDa protein and MAP2 are immunologically related. To study the relationship between the 72‐kDa protein and MAP2C, a smaller molecular form of MAP2 identified in juvenile rat brain, we prepared the 72‐kDa protein from rat brain by the same method as that used for porcine brain. The fact that the 72‐kDa protein from juvenile rat brain was also stained with our monoclonal anti‐MAP2 antibody also suggested that the 72‐kDa protein is an MAP2C homologue of the porcine brain.