We previously prepared cell lines that inducibly overexpress MAP4, a microtubule (MT)-associated protein widely expressed in non-neuronal cells. Overexpression of either the full-length MAP4 molecule or its MT-binding domain, MTB, stabilized MTs and retarded cell growth, suggesting that overexpressed MAP4 impacts on MT-dependent functions in vivo. To test this hypothesis, we examined MT-based vesicle movements in living cells, using high resolution DIC microscopy. Overexpression of either MAP4 or MTB yielded a dose-dependent reduction in the frequency of MT-dependent organelle movements, relative to control cells. At steady state, both MAP4- and MTB-overexpressing cells showed unusual distributions of transferrin, LDL, dextran, and Golgi elements, as compared to control cells. MAP4 preferentially inhibited receptor-dependent uptake and degradation of LDL, and repositioning of Golgi elements after disruption by the drug, brefeldin A. L-MOCK cells treated with Taxol to stabilize the MTs to an extent equivalent to MAP4 overexpression did not show similar inhibition of vesicle motility or organellar trafficking, suggesting that deficits in organelle movements in vivo represent a direct effect of the presence of MAP4 or MTB, rather than an indirect effect of the stabilization of MTs by overexpressed MAP constructs. Our results show that MAP4 has the capacity to affect transport along MTs in vivo; these findings suggest a potential mechanism by which MAP4 could contribute to polarization or morphogenesis of cells.

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