Figure 1.

Formation of tubular membrane networks in response to BFA treatment. Rat liver Golgi fraction membranes were incubated in interphase Xenopus egg cytosol in the presence and absence of BFA and were observed by VE-DIC. (A) Left panel, typical VE-DIC image of rat liver Golgi membranes incubated in interphase cytosol for 60 min, in the absence of BFA. Microtubules (closed arrowhead) polymerize from endogenous Xenopus tubulin, and membrane vesicles (open arrowheads) are observed moving along them. Large membrane clumps (open arrow) are essentially nonmotile, although membrane tubules are occasionally seen extending from them (closed arrow). In the presence of 100 μg/ml BFA (right panel), an intricate and extended tubular membrane network forms. Bar, 5 μm (applies to both panels). (B) Microtubule motor activity of BFA-induced membrane tubules. A sequence of VE-DIC images taken at 5 s intervals shows a typical BFA-induced membrane tubule (arrow) moving along a microtubule (arrowhead). Note that the moving membrane tubule tip does not possess the characteristic globular domain seen in previous studies using different cytosol (Allan and Vale, 1994). Bar, 5 μm (applies to all three panels).