Organization of microtubules in centrosome-free cytoplasm.

Abstract

Abstract. Many different cell types possess microtubule patterns which appear to be polarized and oriented, in part, by cytoplasmic factors not directly associated with a centrosome. Recently, we demonstrated that cytoplasmic extensions ("arms") of teleost melanophores will reorganize their microtubule population outward from their centers after surgical isolation (McNiven, M. A., M. Wang, and K. R. Porter. 1984. Cell. 37:753-765). In the study reported here, we examine microtubule dynamics within the centrosomefree fragments and find that, after severing, microtubule reorganization is initiated at the proximal (cut) end of an arm and migrates distally with the aggregated pigment mass until it becomes permanently positioned at the middle of the arm. Computer-aided image analysis demonstrates that this middle position is located at the arm centroid, implicating the action of a cytoplasmic gel in this process. Morphological studies of arms devoid of pigment reveal that microtubules do not emanate from a single site or structure within the centroid area, but from a more generalized region. Taken together, these findings suggest that factors distributed throughout cytoplasm participate in microtubule assembly and organization.

Encounters between Dynamic Cortical Microtubules Promote Ordering of the Cortical Array through Angle-Dependent Modifications of Microtubule Behavior[W]

“…Computer simulations of microtubule array organization show that microtubule ordering requires the modification of the stochastic behavior of microtubules through the action of external factors such as motor proteins (Vorobjev et al, 2001;Nedelec, 2002;Cytrynbaum et al, 2004), pigment granules (McNiven and Porter, 1988;Vorobjev et al, 2001), or geometric constraints (Maly and Borisy, 2002). In the case of higher plant cells, motor proteins are unlikely to be involved in the parallel organization of cortical microtubules because motor-driven cortical microtubule translocation has not been detected (Shaw et al, 2003;Vos et al, 2004).…”

Computational model of dynein-dependent self-organization of microtubule asters

“…Molecular motors also play an important yet poorly understood role in the organization of MT arrays (Sharp et al, 2000). Remarkably, polar MT arrays can self-organize in the absence of centrosomes (McNiven and Porter, 1988;Maniotis and Schliwa, 1991;Verde et al, 1991). For example, in mitotic extracts, aggregation of MT minus ends is accomplished by large complexes consisting of multiple cytoplasmic dynein motors, the dynein-activator dynactin and the large protein NuMa (Verde et al, 1991).…”

A Kinesin Mutant with an Atypical Bipolar Spindle Undergoes Normal Mitosis

“…Motor proteins, which hydrolyze ATP and move along microtubules, are key players in this reorganization process (Merdes and Cleveland, 1997;Waters and Salmon, 1997;Wittmann et al, 2001). In a centrosome-free environment, motor proteins can induce the formation of microtubule asters (McNiven and Porter, 1988;Maniotis and Schliwa, 1991;Verde et al, 1991;Nedelec et al, 1997). In particular, in acentrosomal Xenopus egg extracts, the minus-end directed microtubule motor, dynein, focuses microtubule minus-ends into spindle poles (Heald et al, 1996(Heald et al, , 1997 and the plus-end directed kinesin, Eg5, is necessary for the establishing a bipolar array (Walczak et al, 1998).…”