Microtubule-associated protein 3 (MAP3) expression in non-neuronal tissues

Microtubule-associated protein 3 (MAP3, Mr 180,000), which in previous studies has been shown to be associated with glial processes and neurofilament-rich axons in rat brain, was examined in various non-neuronal rat tissues. Immunoblots of adult rat tissues (brain, liver, heart, spleen, adrenal medulla and kidney) showed that MAP3 is present in all organs tested. In addition we demonstrated that MAP3 is a heat-stable protein. Using immunohistochemistry, we established the localisation of MAP3 in various cell types. MAP3-containing cells appeared to have in common an asymmetric morphology with long processes that need structural support. In kidney MAP3 is limited to epithelial podocytes and in liver to Kupffer cells. In the adrenal gland, the cells of the cortex are devoid of MAP3 compared to the cells of the medulla. High concentrations of MAP3 are also found in cardiac muscle along the Z-disc and in the smooth muscle cells of the digestive tract. In spleen MAP3 is found in cells of the white pulp surrounding central blood vessels. A co-distribution of MAP3 with microtubules and intermediate filaments but not with microfilaments was found in each cell type examined. The widespread distribution pattern of MAP3 together with its molecular size and heat-stability indicate that MAP3 might be a member of the recently postulated family of homologous 200,000 Mr mammalian tissue MAPs. Potential functions for MAP3 in specific cell types are discussed.