Other ubiquitylated endocytic proteins
In addition to
ubiquitylation of UIM-containing endocytic proteins, a number of other proteins
of the endocytic machinery were described to undergo ubiquitylation. The list
of examples of substrates/E3 pairs is extending, but clear ideas about the
corresponding functions of this modification is often lacking. Examples include
once more proteins invi-olved in the internalization step or in later endocytic
steps. We already described the link between Rsp5p and endocytic proteins
important for the organization of the actin cytoskeleton, and the function
still undefined of Rvs167/amphiphysin ubiquitylation [107]. Amphiphysin displays domains (SH3, BAR domain) and
properties similar to that of endophilin. A specific endophilin isoform,
endophilin A1, was described to undergo ubiquitylation by the Nedd4-like
protein Itch, probably at endosomes [184].
Agonist-dependent
internalization of G protein-coupled receptors via clathrin-coated pits is dependent on the adaptor protein b-arrestin,
which interacts with elements of the endocytic machinery such as AP2 and
clathrin. For the b2-adrenergic receptor (b2-AR) this
requires ubiquitylation of b-arrestin by monomeric RING E3, Mdm2. Although the
receptor is ubiquitylated, it is b-arrestin ubiquitylation which is critical for its
internalization. Two distinct patterns of b-arrestin
trafficking within the cell have been observed for different classes of GPCRs.
For class A (Ex : b2AR), b-arrestin interacts transiently with the receptor at
the cell surface, and for class B (Ex : vasopressin receptor V2R), b-arrestin
displays simultaneous trafficking with the receptors from the cell membrane to
endocytic vesicles. This differential behavior was shown to rely on different
ubiquitylation/deubiquitylation patterns of b-arrestin
during trafficking of these two class of receptors. b2AR stimulation
leads to only transient b-arrestin ubiquiitylation, whereas V2R stimulation
leads to stable b-arrestin ubiquitylation Expression of
arrestin-ubiquitin chimera transforms a class A receptor to class B with
respect to intracellular trafficking. One possible interpretation could be that
persistently ubiquitylated b-arrestin in complex with internalized receptors
somehow leads to their MVB sorting, instead of recycling [185]. Mechanisms underlying regulation of b-arrestin
deubiquitylation have still to be defined.
Similar
features prevail in the case of EGFR internalization. In the CIN85-endophilin-Cbl
complex required for EGFR endocytosis, CIN85 itself is massively
monoubiquitylated by Cbl after EGF treatment [186]. Both Cbl and CIN85 are then targeted to MVBs and
degraded together with EGFR. Strikingly, Cbl-directed CIN85 monoubiquitylation
is not specific of a particular Lys residue. Dominant negative forms of CIN85
which delay EGFR degradation display impaired monoubiquitylation, indicating
that this modification plays a critical role in EGFR endocytic pathway, likely
for both internalization, and MVB sorting. These data are rather similar to
that reported for b2-AR and b-arrestin, since in both case there is a ligand
stimulated ubiquitylation simultaneously for the endocytic cargoes, and for a
corresponding adaptor. In addition,
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