Figure 9.

Model for interphase MT bundle generation. A perinuclear and free cytoplasmic pool of γ-TuCs coexists and possibly exchanges. Cytoplasmic γ-TuCs bind to existing MTs (1) and move toward the nucleus during MT catastrophes (2) or are moved to the nucleus with an adhered nucleated MT (3). New MTs originate primarily from the perinuclear pool and first grow out as a single MT (4). Perinuclear nucleation may secure a nuclear attachment of the iMTOC that is under construction. A bipolar bundle is formed when a γ-TuCs adheres to a new MT and nucleates a second MT in an antiparallel manner (5). Two minus ends are moved towards each other by a mechanism that likely involves MT minus-end–directed motors and bundling proteins. A region of MT overlap is maintained that can bind to the nuclear membrane (6). In our model γ-TuCs demarcate regions of MT overlap as is observed (Fig. 8 C). The motion of small MTs along MT bundles largely explains the previously reported complex motion of MT associated proteins, like tip1p (CLIP-170) and mal3p (EB1; Busch and Brunner, 2004).