Separable roles for RanGTP in nuclear and ciliary trafficking of a kinesin-2 subunit

Kinesin is part of the microtubule-binding motor protein superfamily, which serves important roles in cell division and intraorganellar transport. The heterotrimeric kinesin-2, consisting of the heterodimeric motor subunits, kinesin family member 3A/3B (KIF3A/3B), and kinesin-associated protein 3 (KAP3), is highly conserved across species from the unicellular eukaryote Chlamydomonas to humans. It plays diverse roles in cargo transport including anterograde (base to tip) trafficking in cilia. However, the molecular determinants mediating trafficking of heterotrimeric kinesin-2 itself are poorly understood. It has been previously suggested that ciliary transport is analogous to nuclear transport mechanisms. Using Chlamydomonas and human telomerase reverse transcriptase-retinal pigment epithelial cell line, we show that RanGTP, a small GTPase that dictates nuclear transport, regulates ciliary trafficking of KAP3, a key component for functional kinesin-2. We found that the armadillo-repeat region 6 to 9 (ARM6-9) of KAP3, required for its nuclear translocation, is also necessary and sufficient for its targeting to the ciliary base. Given that KAP3 is essential for cilium formation and there are the emerging roles for RanGTP/importin β in ciliary protein targeting, we further investigated the effect of RanGTP in cilium formation and maintenance. We found that precise control of RanGTP levels, revealed by different Ran mutants, is crucial for cilium formation and maintenance. Most importantly, we were able to provide orthogonal support in an algal model system that segregates RanGTP regulation of ciliary protein trafficking from its nuclear roles. Our work provides important support for the model that nuclear import mechanisms have been co-opted for independent roles in ciliary import.

The heterotrimeric kinesin-2 complex interacts with and regulates GLI protein function

GLI transport to the primary cilium and nucleus is required for proper Hedgehog (HH) signaling; however, the mechanisms that mediate these trafficking events are poorly understood. Kinesin-2 motor proteins regulate ciliary transport of cargo, yet their role in GLI protein function remains unexplored. To examine a role for the heterotrimeric KIF3A-KIF3B-KAP3 kinesin-2 motor complex in regulating GLI activity, we performed a series of structure-function analyses using biochemical, cell signaling and in vivo approaches that define novel specific interactions between GLI proteins and two components of this complex, KAP3 and KIF3A. We find that all three mammalian GLI proteins interact with KAP3 and we map specific interaction sites in both proteins. Furthermore, we find that GLI proteins interact selectively with KIF3A, but not KIF3B, and that GLI interacts synergistically with KAP3 and KIF3A. Using a combination of cell signaling assays and chicken in ovo electroporation, we demonstrate that KAP3 interactions restrict GLI activator function but not GLI repressor function. These data suggest that GLI interactions with KIF3A-KIF3B-KAP3 complexes are essential for proper GLI transcriptional activity.

Overexpression of Kinesin Associated Protein 3 (KIFAP3) in Breast Cancer

Gene expression profiling studies on breast cancer have generated catalogs of differentially expressed genes. However, many of these genes have not been investigated for their expression at the protein level. It is possible to systematically evaluate such genes in a high-throughput fashion for their overexpression at the protein level using breast cancer tissue microarrays. Our strategy involved integration of information from publicly available repositories of gene expression to prepare a list of genes upregulated at the mRNA level in breast cancer followed by curation of the published literature to identify those genes that were not tested for overexpression at the protein level. We identified Kinesin Associated Protein 3 (KIFAP3) as one such molecule for further validation at the protein level. Immunohistochemical labeling of KIFAP3 using breast cancer tissue microarrays revealed overexpression of KIFAP3 protein in 84% (240/285) of breast cancers indicating the utility of our integrated approach of combining computational analysis with experimental biology.