F‐actin bundling protein from Physarum polycephalum: Purification and its capacity for co‐bundling of actin filaments and microtubules

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Abstract
An F‐actin bundling protein was isolated and purified from plasmodium of Physarum polycephalum. The F‐actin bundling protein in Physarum extract was passed through a DEAE‐cellulose column. After the protein in the fraction was treated with 6 M urea, it was purified by gel filtration on Sephacryl S‐300 HR followed by chromatography on CM‐Toyopearl (cation exchange) in the presence of 6 M urea. The purified protein gave a single band on SDS‐PAGE, and the molecular weight was estimated to be 52,000. This F‐actin bundling protein is referred to as the 52 kDa protein.Interestingly, the 52 kDa protein also induced bundling of microtubules. The formation of F‐actin and microtubule bundles was Ca2+‐insensitive, but depended on the salt concentration. Each bundle formed at NaCl concentrations less than 0.1 M. The 52 kDa protein cross‐reacted with monoclonal antibody raised against a HeLa 55 kDa protein (an F‐actin bundling protein from HeLa cells) (Yamashiro‐Matsumura and Matsumura: J. Biol. Chem. 260:5087‐5097, 1985).When the 52 kDa protein was added to a mixture of actin filaments and microtubules, co‐bundles composed of both filaments formed. This is the first reported example in which an F‐actin bundling protein induced co‐bundling of actin filaments and microtubules.

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