Role of WDR35 in the formation of functional cilia
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Date
27/06/2020Item status
Restricted AccessEmbargo end date
27/06/2021Author
Quidwai, Tooba
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Abstract
Cilia are microtubule-based organelles present on the surface of almost all mammalian
cells that play key sensory and sometimes motile functions. Defects in cilia structure
or function lead to a group of human diseases called the ciliopathies. In order to
function, cilia must maintain a distinct protein and membrane composition from the
surrounding plasma membrane and cytosol, highly enriched in signaling receptors and
effectors. How this compartmentalization occurs remains unclear. There is no known
protein synthesising machinery in cilia and the transition zone at the base of cilium
forms the diffusion barrier, which does not allow free exchange between the cytoplasm
to cilioplasm. Biogenesis of the cilium (ciliogenesis) requires many carriers and
adaptors to facilitate passage of cargo across the transition zone. Once inside the cilia
intraflagellar transport (IFT) proteins move the cargo along microtubules ‘railways’ of
the axoneme. IFTs are known to assemble in two protein complexes, IFTA and IFTB
protein complexes. IFT-B complex known to be made of 16 different proteins mediate
anterograde transport with the help of kinesin motors, and IFT-A complex made of 6
different proteins help in retrograde transport powered by dynein motors. However,
the exact mechanism of transport of cargo to cilia and entry across the diffusion barrier
is not well understood, and the functions of each IFT protein remain unclear. In this
thesis, I describe the critical role of one of the IFT-A complex protein WDR35/IFT121
in the formation of functional cilia by transporting structural elements of cilia via a
vesicular mediated pathway.
Null mutations in the IFT-A component WDR35/ IFT121 are embryonic lethal in
both mouse models and human ciliopathies (Mill et al., 2011). Small, unstable WDR35
mutant cilia are formed but fail to become enriched in diverse classes of integral and
membrane-associated proteins (Caparrós-Martín JA et al., 2015; Fu W et al., 2016).
To elucidate its role in the entry of membrane proteins to cilia, I present live and fixed
cell imaging experiments to visualize the dynamics of membrane protein localization
at the periciliary base. I also performed interaction studies by immunoprecipitation and
mass spectrometry to define the molecular mechanism by which IFT proteins establish
functional cilia.
There are few reports of some IFTs having similarity to COPI, II and clathrin vesicles
(Jékely G and Arendt D, 2006; Taschner M et al., 2012; Dam TJPV et al., 2013). To
further explore its role, I performed deep sequence searches and homology modeling
of the entire IFTA complex. I found that three of its core components and WDR35
have structural homology to COPI complex proteins α and ß’. To test whether WDR35
could function as a coatomer, I performed transmission electron microscopy
tomography on cilia mutants. I found that in contrast to the electron-dense vesicles
observed around WT cilia, Wdr35-/- cilia had a ten-fold increase in the number of
vesicles all lacking this outer electron density. This suggests WDR35 may be involved
in coating cilia-bound vesicles in order to transfer cargo into cilia, functioning similar
to COPI complex proteins which selectively transports cargo between the endoplasmic
reticulum and Golgi.