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Abstract:
Cilia/flagella are microtubule (MT) based membrane protrusions present on the
surface of many eukaryotic cells. Once regarded as vestigial organelles without much
functionality, cilia are now acknowledged to play critical roles in many aspects of
human development, health and disease phenotypes through functions in cellular
motility, signaling and sensory pathways. Cilia exist in many forms and serve a wide
variety of tissue-specific functions. Given that cilia do not contain ribosomes and
ciliary components are constantly turning over at the tip, proteins destined for cilia are
synthesized in the cytoplasm and have to be actively transported into the cilium.
Almost all cilia rely on a dedicated transport process called intraflagellar transport
(IFT) for assembly and maintenance. IFT involves a multi-protein mega-dalton
complex, known as the IFT complex that binds ciliary cargoes and transports them
into and out of the cilium. IFT proteins are highly conserved in ciliated organisms
ranging from the unicellular biflagellate green alga Chlamydomonas reinhardtii
(Chlamydomonas or Cr) to higher eukaryotes including humans. Mutations in IFT
genes lead to severe defects in cilia formation and are the primary cause of many
ciliopathies (diseases caused by ciliary dysfunction). Chlamydomonas is an important
model organism to study ciliary assembly and major discoveries such as the process
of IFT, purification of IFT particles and tubulin turnover were made using simple but
elegant experiments in this biflagellate. Although IFT is implicated in the transport of
major axonemal components such as tubulin and outer dynein arms, a direct link
between an IFT protein and a ciliary cargo was still missing prior to this study, largely
due to the inadequate characterization of individual IFT proteins. This thesis describes
the structural and biochemical characterization of 4 IFT proteins - namely IFT27,
IFT25, IFT74 and IFT81 - from human and/or Chlamydomonas.
