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Fig. 8. Scheme of MT organization and nuclear migration in A. nidulans. (A) Cytoplasmic MTs are generated from SPBs. They elongate and eventually overlap to form anti-parallel bundles (1), thereby connecting adjacent nuclei. Some grow tipwards or make contact with the cortex (2). Nuclei can be moved by pulling forces of motor proteins, which could be located between overlapping filaments (1) or at the cortex (2), as the right nucleus indicates. (B) In wild-type cells, MTs contact ApsA or associated proteins (a). In apsB mutant cells, only a dominant, central microtubule bundle passes through the compartment, while additional MTs are absent (b). Although present, MTs cannot successfully contact the cortex in apsA mutant cells, hence elongating, which results in the appearance of curved MTs (c). Because certain MTs are not present in apsB mutants, nuclear positioning is interrupted, while MTs in apsA mutants miss the interaction with ApsA. Therefore, apsA and apsB mutants show very similar nuclear migration defects. Although cortical pulling forces (2) may be ineffective here, pulling forces between overlapping filaments (1) probably still work and are responsible for the observed remaining nuclear migration activity.
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