Cell News 01/2017
24
Landscape of nuclear transport receptor cargo specificity
Marie-Therese Mackmull, Alessandro Ori, Martin Beck
SESSION 2: MECHANISMS OF NUCLEOCYTOPLASMIC EXCHANGE
Presenting author: Marie-Therese Mackmull
Structural and Computational Biology, EMBL, Heidelberg
The main function of the nuclear pore complex (NPC) is to
facilitate and regulate the transport between the cytosol and
the nucleus but NPCs are also involved in various other cellular
functions, including protein synthesis, signal transduction and
cell differentiation. The nucleocytoplasmic transport system
is composed of the NPC itself, that forms a large aqueous
channel lined with FG-repeats containing nucleoporins (Nups).
FG-Nups constitute a permeability barrier, which prevents
the passage of the majority of all proteins. Nuclear transport
receptors (NTRs, also called importins or exportins) specifically
recognize localization signals of cargos and facilitate their pas-
sage through the NPC by transiently interacting with FG-Nups.
Classical methods such as affinity purification or measurement
of dissociation constants are not well-suited to globally identi-
fy NTR-cargo interactions because they are of a very transient
nature, the spectrum of recognized cargos is huge and their
dynamic concentration range comprises orders of magnitude.
The exact cargo spectrum of the majority of NTRs, their spec-
ificity and even the extent to which active nucleocytoplasmic
transport contributes to protein localization thus remains
uncertain. To systematically map cargo-NTR relationships in an
unbiased way in vivo, we used proximity labeling mass spec-
trometry based on the so-called BioID system. We systemati-
cally fused BirA to various NTRs and other factors involved in
nucleocytoplasmic transport. We found that at least one third
of the human proteome is subject to active nuclear transport.
We characterized the specific cargo spectrum of several NTRs
and estimated their specificity or overlap with other transport
pathways. We identified the responsible transport pathways
of various key protein complexes and demonstrate that those
and components of pathways tend to be transported by related
NTRs. The identification of the exact biotinylation sites pro-
vides evidence for the relevant interaction surfaces and shades
light in direct versus piggyback transport mechanisms.
This work was funded by EMBL and the European Research
Council through grant No. 309271 (NPCAtlas) given to M.B.
