Expanding the Role of the Dynein Regulatory Complex to Non-Axonemal Functions: Association of GAS11 with the Golgi Apparatus

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Abstract
The mammalian GAS11 gene is a candidate tumor suppressor of unknown function that was previously identified as one of several genes upregulated upon growth arrest. Interestingly, although GAS11 homologs in Trypanosoma brucei (trypanin) and Chlamydomonas reinhardtii (PF2) are integral components of the flagellar axoneme and are necessary for regulating flagellar beat, the GAS11 gene was discovered based on its expression in cells that do not assemble a motile cilium. This suggests that GAS11 function might not be restricted to the cilium. To investigate this possibility, we generated GAS11-specific antibodies and demonstrate here that GAS11 is expressed in a variety of mammalian cells that lack a motile cilium. In COS7 cells, GAS11 is associated with the detergent-insoluble cytoskeleton and exhibits a juxtanuclear localization that overlaps with the pericentrosomal Golgi apparatus. This localization is dependent upon intact microtubules and is cell-cycle regulated, such that GAS11 is dispersed throughout the cytoplasm as cells progress through mitosis. GAS11 remains associated with Golgi fragments following depolymerization of cytoplasmic microtubules but is dispersed upon disruption of the Golgi with brefeldin A. These data suggest that GAS11 is associated with the Golgi apparatus. In support of this, recombinant GAS11 binds Golgi membranes in vitro. In growth-arrested mIMCD3 cells, GAS11 co-localizes with γ-tubulin at the base of the primary cilium. The pericentrosomal Golgi apparatus and base of the cilium both represent convergence points for microtubule minus ends and correspond to sites where dynein regulation is required. The algal GAS11 homolog functions as part of a dynein regulatory complex (DRC) in the axoneme (Rupp and Porter. J Cell Biol 2003;162:47–57) and our findings suggest that components of this axonemal dynein regulatory system have been adapted in mammalian cells to participate in non-axonemal functions.