Cilia and flagella (interchangeable terms) are evolutionarily conserved organelles found on many different types of eukaryotic cells where they fulfill important functions in motility, sensory reception and signaling. The process of Intraflagellar Transport (IFT) is of central importance for both the assembly and maintenance of cilia, as it delivers building blocks from their site of synthesis in the cell body to the ciliary assembly site at the tip of the cilium. A key player in this process is the multi-subunit IFT-complex, which acts as an adapter between the motor proteins required for movement and the ciliary cargo proteins. Since the discovery of IFT more than 15 years ago, considerable effort has gone into the purification and characterization of the IFT complex proteins. Even though this has led to very interesting findings and has greatly improved our knowledge of the IFT process, we still know very little about the overall architecture of the IFT complex and the specific functions of the various subunits. In this review we will give an update on the knowledge of the structure and function of individual IFT proteins, and the way these proteins interact to form the complex that facilitates IFT.
"Cilia are microtubule-based protrusions arising from a modified centriole: the basal body (Pazour and Witman, 2003). Cilium biogenesis and function require a complex network of cilia-associated proteins that serve, for example, to anchor the basal body to the cell membrane or to selectively allow protein entry and exit, thus creating a distinct subcellular compartment (Gherman et al., 2006; Ishikawa and Marshall, 2011; Nachury et al., 2010; Pazour and Witman, 2003; Pedersen et al., 2008; Reiter et al., 2012; Taschner et al., 2012). The ability to function as a separate compartment appears key to the primary cilium acting as an environmental sensor and signaling hub (Berbari et al., 2009). "
[Show abstract][Hide abstract] ABSTRACT: The development of multicellular organisms requires the precisely coordinated regulation of an evolutionarily conserved group of signaling pathways. Temporal and spatial control of these signaling cascades is achieved through networks of regulatory proteins, segregation of pathway components in specific subcellular compartments, or both. In vertebrates, dysregulation of primary cilia function has been strongly linked to developmental signaling defects, yet it remains unclear whether cilia sequester pathway components to regulate their activation or cilia-associated proteins directly modulate developmental signaling events. To elucidate this question, we conducted an RNAi-based screen in Drosophila non-ciliated cells to test for cilium-independent loss-of-function phenotypes of ciliary proteins in developmental signaling pathways. Our results show no effect on Hedgehog signaling. In contrast, our screen identified several cilia-associated proteins as functioning in canonical Wnt signaling. Further characterization of specific components of Intraflagellar Transport complex A uncovered a cilia-independent function in potentiating Wnt signals by promoting β-catenin/Armadillo activity.
"Because no complex was formed between IFT22 and IFT27/25 (Fig. S5 B), we concluded that IFT22 binds directly to the IFT81/74 subcomplex. IFT81 and IFT74 contain predicted coiled-coil domains that mediate the interactions between the two proteins as well as central linker regions that likely divide the proteins into N-terminal and C-terminal coiled-coil domains (Lucker et al., 2005; Taschner et al., 2012). Whereas full-length IFT81/74 or the C-terminal coiled-coil domains were not soluble when recombinantly expressed, the N-terminal coiled-coil domains with or without the linker regions as well as the short linker regions alone were soluble and could be assayed for interaction with IFT22. "
[Show abstract][Hide abstract] ABSTRACT: Cilia are microtubule-based organelles that assemble via intraflagellar transport (IFT) and function as signaling hubs on eukaryotic cells. IFT relies on molecular motors and IFT complexes that mediate the contacts with ciliary cargo. To elucidate the architecture of the IFT-B complex, we reconstituted and purified the nonameric IFT-B core from Chlamydomonas reinhardtii and determined the crystal structures of C. reinhardtii IFT70/52 and Tetrahymena IFT52/46 subcomplexes. The 2.5-Å resolution IFT70/52 structure shows that IFT52330-370 is buried deeply within the IFT70 tetratricopeptide repeat superhelix. Furthermore, the polycystic kidney disease protein IFT88 binds IFT52281-329 in a complex that interacts directly with IFT70/IFT52330-381 in trans. The structure of IFT52C/IFT46C was solved at 2.3 Å resolution, and we show that it is essential for IFT-B core integrity by mediating interaction between IFT88/70/52/46 and IFT81/74/27/25/22 subcomplexes. Consistent with this, overexpression of mammalian IFT52C in MDCK cells is dominant-negative and causes IFT protein mislocalization and disrupted ciliogenesis. These data further rationalize several ciliogenesis phenotypes of IFT mutant strains.
The Journal of Cell Biology 10/2014; 207(2):269-82. DOI:10.1083/jcb.201408002 · 9.83 Impact Factor
"The identification of ASH/MSP domains in the TRAPPII subunits underpins their previously proposed function in ciliary membrane biogenesis  at the molecular level, and corroborates the idea that the ASH domain is associated with cilia-related functions . The presence of an amino terminal α-helical TPR repeat region is also a hallmark of numerous ciliary proteins , and hence the presence of such a TPR repeat region in the N-terminus of most TRAPPII subunits (Figure 1) is in line with their ciliary function. As with the ASH domain, the TPR repeat region also seems to be functionally important because mutation leading to deletion of residues 372-429 of TRAPPC11 were shown to impair post-Golgi trafficking and to cause myopathy, infantile hyperkinetic movements, ataxia and intellectual disability in patients . "
[Show abstract][Hide abstract] ABSTRACT: Background
Assembly of primary cilia relies on vesicular trafficking towards the cilium base and intraflagellar transport (IFT) between the base and distal tip of the cilium. Recent studies have identified several key regulators of these processes, including Rab GTPases such as Rab8 and Rab11, the Rab8 guanine nucleotide exchange factor Rabin8, and the transport protein particle (TRAPP) components TRAPPC3, -C9, and -C10, which physically interact with each other and function together with Bardet Biedl syndrome (BBS) proteins in ciliary membrane biogenesis. However, despite recent advances, the exact molecular mechanisms by which these proteins interact and target to the basal body to promote ciliogenesis are not fully understood.
We surveyed the human proteome for novel ASPM, SPD-2, Hydin (ASH) domain-containing proteins. We identified the TRAPP complex subunits TRAPPC8, -9, -10, -11, and -13 as novel ASH domain-containing proteins. In addition to a C-terminal ASH domain region, we predict that the N-terminus of TRAPPC8, -9, -10, and -11, as well as their yeast counterparts, consists of an α-solenoid bearing stretches of multiple tetratricopeptide (TPR) repeats. Immunofluorescence microscopy analysis of cultured mammalian cells revealed that exogenously expressed ASH domains, as well as endogenous TRAPPC8, localize to the centrosome/basal body. Further, depletion of TRAPPC8 impaired ciliogenesis and GFP-Rabin8 centrosome targeting.
Our results suggest that ASH domains confer targeting to the centrosome and cilia, and that TRAPPC8 has cilia-related functions. Further, we propose that the yeast TRAPPII complex and its mammalian counterpart are evolutionarily related to the bacterial periplasmic trafficking chaperone PapD of the usher pili assembly machinery.
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