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Alix is depicted as a bridging factor between ESCRT-I and ESCRT-III, but whether Alix binds simultaneously to both complexes is not known.

Reproduced with permission from Elsevier.

The specific role of Alix in ESCRT-mediated protein sorting has yet to be determined.

A role for Alix in this process was initially suggested by observations that the loss of Bro1 function in yeast impairs the downregulation of cell-surface proteins that normally undergo endocytosis and delivery to the vacuole, the functional equivalent of the lysosome in higher eukaryotes ;.

The Bro1 domain from the yeast Bro1 protein.

Similarly, Alix binds to a YPAL motif in the Gag protein of murine leukemia virus, the budding of which is impaired upon RNAi-mediated depletion of Alix.

Collectively, class E Vps proteins function at the cytosolic face of endosomal membranes to sort integral membrane proteins into lumenal vesicles following invagination of the endosomal membrane.

Alix also is enriched in virions that are released from cells under the direction of the HIV-1 Gag protein ;.

Immunofluorescence studies suggest that a similar mechanism is responsible for the recruitment of Alix to endosomes because overexpression of CHMP4b causes accumulation at endosomes of both full-length Alix and a truncated form containing the Bro1 domain.

As a consequence of their recruitment, Rim13 proteolytically cleaves a C-terminal inhibitory domain from Rim101, thereby enabling Rim101 to be released from endosomes and enter the nucleus.

In this regard, it would be interesting to know whether growth factor stimulation triggers the assembly of Alix with ALG-2 on endosomes in order to prime cells for entry into apoptosis.