| Summary: | Degradation of membrane proteins in the vacuole/lysosome is dependent on their prior sorting into the multivesicular body (MVB) pathway. This sorting process involves incorporation of proteins into vesicles that are formed by budding of the limiting membrane of the endosome into the lumen of the organelle. The MVB sorting process on the whole is highly conserved from yeast to human, and depends on the Vps27/Hrs, ESCRT-I, -II, and -III protein complexes functioning sequentially on the endosomal membrane, as well as on additional factors, such as the ubiquitinating enzyme Rsp5/Nedd4. It has now been established that ubiquitin serves as a sorting signal for many cargoes into the MVB pathway.
In this thesis work, we provide evidence that Bro1 is not required for protein ubiquitination or early steps of endocytosis, but functions at the late endosome level as an integral component of the MVB pathway. Similarly to its human homologue Alix, Bro1 interacts with components of the ESCRT-I and ESCRT-III complexes. The putative role of Bro1/Alix in bridging an interaction between ESCRT-I and –III might be important to strengthen an association of these protein complexes to allow efficient sorting of cargo proteins. Deficiency in Bro1 results in recycling of the endocytosed Gap1 permease back to the plasma membrane, a process coupled to deubiquitination of the permease. This recycling is a non-classical phenotype for cells impaired in MVB pathway thus suggesting Bro1 to have a particular role in this sorting process. Furthermore, the conserved C-terminal proline-rich domain (PRD) of Bro1 is specifically important for MVB sorting of cargo proteins that are subject to ubiquitination. We show Bro1 (via its PRD) to play a highly important role in recruitment of the deubiquitinating enzyme Doa4 to the endosome. Consistent with this, Bro1 is required for deubiquitination of cargo proteins, a step occurring just before cargo incorporation into the endosomal vesicles, and similarly to Doa4, for ubiquitin recycling. In contrast to previous interpretations, we show that Doa4 has a direct role in sorting of ubiquitinated cargo proteins into the MVB pathway. We propose that Doa4 – via its association to Bro1 - achieves this role by catalyzing deubiquitination of cargo proteins and/or some components of the MVB sorting machinery. We further show Bro1 to interact with the ubiquitin ligase Rsp5, which, in addition to being required for cargo protein ubiquitination at the plasma membrane, apparently contributes to multiple steps of endocytosis and MVB sorting. Also the Bro1-Rsp5 interaction is dependent on the C-terminal PRD region of Bro1. We propose that this interaction is conserved.
A role for ubiquitin in regulation of the MVB sorting machinery is emerging: the function of factors recognizing and sorting ubiquitinated cargo proteins in the MVB pathway is suggested to be coupled to their cycling between ubiquitinated and deubiquitinated stages. A growing body of evidence indicates that ubiquitin ligases of the Rsp5/Nedd4 family play a central role in this regulation. We speculate the Bro1/Alix protein, through its ability to simultaneously interact with factors of the MVB sorting machinery and with ubiquitinating and deubiquitinating enzymes to play a central role in the successive rounds of ubiquitination and deubiquitination of specific factors along the MVB pathway.
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