MAP2 is a component of crossbridges between microtubules and neurofilaments in the neuronal cytoskeleton: quick-freeze, deep-etch immunoelectron microscopy and reconstitution studies

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Abstract
Microtubules (MT) and neurofilaments (NF) are linked by frequent crossbridges in situ. In order to answer the question of what makes these crossbridges, we performed the immunogold procedure on rat spinal cord motor neurons using an affinity-purified polyclonal antibody against rat brain MAP2 and gold-labeled anti-rabbit IgG goat IgG. A quick-freeze, deep-etch technique (QF-DE) in conjunction with decoration with anti-MAP2 antibody and ferritin-labeled second antibody was also used. In motor neuron dendrites crossbridges were clearly displayed between MTs and NFs by QF-DE. These crossbridges were revealed in thin sections as fuzzy filamentous structures between MT and NF. Gold particles studded the fuzzy structures associated with MT. Many such structures connected MTs to NFs. Furthermore, antibody complexes containing ferritin were localized on the crossbridges between MTs and NFs by the QF-DE study. In addition, we performed reconstitution experiments. We isolated 70 kDa (L) protein of neurofilaments from calf spinal cords and assembled L to form neurofilaments in vitro. MAP2 bound these neurofilaments according to both SDS-PAGE and QF-DE electron microscopy of the pellets of suspensions containing L proteins and MAP2. When we added tubulin to this suspension and polymerized it in the presence of taxol, neurofilaments were crosslinked with microtubules by MAP2 crossbridges. Hence, from these 2 approaches we concluded that MAP2 is a component of crossbridges between MTs and NFs in the neuronal cytoskeleton in vivo and in vitro.

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