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Characterising the function of Ubiquitin associated protein 1 (UBAP1)
[Thesis]. Manchester, UK: The University of Manchester; 2013.
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Abstract
Inactivating EGF signalling is key to modulating cell growth and avoidingcancer. To do this, the EGF receptor is ubiquitinated, internalized and sorted tolysosome for degradation. This latter process is coordinated by the endosomal sortingcomplex required for transport (ESCRT) machinery, a multi-complex proteinmachinery divided into four groups: ESCRT-0, I, II, III. ESCRTs recognise ubiquitinatedcargoes and sort them from the limiting membrane of intermediate vesiclesof maturing endosomes. In mammalian cells, the ESCRT machinery is also involvedin other membrane related events, such as cytokinesis and viral budding. CertainESCRTs, such as ESCRT-0, seem to be specifically required for cargo sorting tolysosomes, whereas other downstream ESCRTs, such as ESCRT-I, are required forall the cellular processes where the ESCRT machinery is involved. The existence ofmultiple variants of ESCRT-I components may suggest that ESCRT-I itself existsin different variants, each specific for a different membrane-based event.A bioinformatic study suggested Ubiquitin Associated Protein1 (UBAP1)as a novel variant of the ESCRT-I component MVB12. Moreover, a preliminaryY2H study identified UBAP1 as a potential binding partner of the ESCRT machineryregulator, HDPTP. This study aims to characterise UBAP1 as a variantMVB12 and a novel member of ESCRT-I. The results show that loss of UBAP1impairs EGFR trafficking to lysosomes and causes the accumulation of ubiquitinatedproteins on aberrant vacuolar structures. In cells, UBAP1 is incorporated ina complex with the ESCRT-I members TSG101, VPS28 and VPS37A. Importantly,UBAP1 uses three tandem Ubiquitin associated (UBA) domains to bind ubiquitinand this activity is key for UBAP1 to function in cells. UBAP1 binds HDPTP viaa peptide motif located about 100 aa. proximal to the tandem UBA domains. Altogether,the data shown in this thesis suggest that UBAP1 represents a subunit ofan endosome-specific ESCRT-I complex, whose function may be coordinated by theESCRT machinery regulator HDPTP.