Membrane/microtubule tip attachment complexes (TACs) allow the assembly dynamics of plus ends to push and pull membranes into tubulovesicular networks in interphase Xenopus egg extracts.

Abstract

Abstract. We discovered by using high resolution video microscopy, that membranes become attached selectively to the growing plus ends of microtubules by membrane/microtubule tip attachment complexes (TACs) in interphase-arrested, undiluted, Xenopus egg extracts. Persistent plus end growth of stationary microtubules pushed the membranes into thin tubules and dragged them through the cytoplasm at the ~20 p,m/ min velocity typical of free plus ends. Membrane tubules also remained attached to plus ends when they switched to the shortening phase of dynamic instability at velocities typical of free ends, 50-60 ixm/min. Over time, the membrane tubules contacted and fused with one another along their lengths, forming a polygonal network much like the distribution of ER in cells. Several components of the membrane networks formed by TACs were identified as ER by immunofluorescent staining using antibodies to ER-resident proteins. TAC motility was not inhibited by known inhibitors of microtubule motor activity, including 5 mM AMP-PNP, 250 IxM orthovanadate, and ATP depletion. These results show that membrane/microtubule TACs enable polymerizing ends to push and depolymerizing ends to pull membranes into thin tubular extensions and networks at fast velocities.

Dynamics and inheritance of the endoplasmic reticulum

“…ER tubules and microtubules closely align over considerable distances and single-point attachment sites are evident at the light microscopic and ultrastructural levels (Baumann and Waltz, 2001). Video microscopy studies have revealed extension of ER tubules along microtubules during ER expansion in live cells and during reconstitution of the ER network from microsomes in cell-free systems (Terasaki et al, 1986;Dabora and Sheetz, 1988;Lee et al, 1989;Allan and Vale, 1991;Allan, 1995;Waterman-Storer et al, 1995;Waterman-Storer and Salmon, 1998). Interestingly, microtubule-dependent motility exclusively controls the rapid extension of ER tubules out to the cell periphery, whereas a slower movement of ER tubules towards the cell center is independent of microtubules (Terasaki and Reese, 1994;Waterman-Storer and Salmon, 1998).…”

“…Microtubule-dependent ER movement results from the activity of microtubule-associated motors as well as the polymerization of microtubules (Dabora and Sheetz, 1988;Vale and Hotani, 1988;Dailey and Bridgeman, 1989;Waterman-Storer et al, 1995;Waterman-Storer and Salmon, 1998;Lane and Allan, 1999). Motors can either drag membranes along underlying microtubules or drive the sliding of membrane-associated microtubules, whereas microtubule polymerization can promote the movement of an ER tubule bound to the dynamic microtubule tip (Waterman-Storer and Salmon, 1998).…”

“…By contrast, ER tubules in Xenopus egg extracts move exclusively towards the minus ends of microtubules, driven by the activity of membrane-associated cytoplasmic dynein (Allan and Vale, 1991;Allan, 1995;Waterman-Storer et al, 1995;Niclas et al, 1996;Steffen et al, 1997). Cytoplasmic dynein, but not conventional kinesin, is required for cortical ER dynamics and reformation in the fungus U. maydis (WedlichSoldner et al, 2002).…”

Posttranslational modification of tubulin by palmitoylation: I. In vivo and cell-free studies.

“…Waterman-Storer et al (1995) presented in vitro studies showing that membranes from Xenopus eggs are moved and positioned after selective attachment to the growing ends of microtubules. Whether such events occur in vivo is not yet known.…”

Mitotic Organization and Force Generation by Assembly/Disassembly of Microtubules.

“…Before attaching to the microtubule plus end, the kinetochore attaches and glides rapidly along the wall of the microtubule (A, right). (After 10) A METAPHASE B METAPHASE (Mitchison et al, 1986;Wise et al, 1991) Evidence for Force Generation by Assembly/Disassembly of Microtubules In addition to governing microtubule length and inducing chromosomesto movetowards or away from the spindle pole in vivo, experiments performed on in vitro systems over the last few years clarify the situation further. They show that assembling and elongating microtubules can push, and that disassembling and shortening microtubules can pull, organelles and other loads (Figs.…”