Evidence for expression of some microtubule-associated protein 1B in neurons as a plasma membrane glycoprotein

Microtubule-associated protein (MAP) 1B is a high-molecular-weight cytoskel etal protein that is abundant in developing neuronal processes and appears to be necessary for axonal growth. Various biochemical and immunocytochemic al results are reported, indicating that a significant fraction of MAP1B is expressed as an integral membrane glycoprotein in vesicles and the plasma membrane of neurons. MAP1B is present in microsomal fractions isolated from developing rat brain and fractionates across a sucrose gradient in a manne r similar to synaptophysin, a well-known vesicular and plasma membrane prot ein. MAP1B is also in axolemma-enriched fractions (AEFs) isolated from myel inated axons of rat brain. MAP1B in AEFs and membrane fractions from cultur ed dorsal root ganglion neurons (DRGNs) remains membrane-associated followi ng high-salt washes and contains sialic acid. Furthermore, MAP1B in intact DRGNs is readily degraded by extracellular trypsin and is labeled by the ce ll surface probe sulfosuccinimidobiotin. Immunocytochemical examination of DRGNs shows that MAP1B is concentrated in vesicle-rich varicosities along t he length of axons. Myelinated peripheral nerves immunostained for MAP1B sh ow an enrichment at the axonal plasma membrane. These observations demonstr ate that some of the MAP1B in developing neurons is an integral plasma memb rane glycoprotein.