is functionally and physically associated with
ABP1 at the cell surface to regulate auxin- and
ABP1-mediated activation of ROP GTPase sig-
naling. TMK1 is at least one of the long-sought
docking proteins coupling extracellular auxin
and its perception by ABP1 to cytoplasmic signal-
ing (6, 14, 24). This discovery solves the mystery
of the cell surface–cytoplasmic auxin perception
and signaling system and opens up a new hori-
zon in auxin biology. Clearly, the TIR1/AFB-
based nuclear pathways are essential for various
auxin responses (1, 2). The pleiotropic pheno-
types of the tmk and abp1 mutants also indicate
an essential role for the extracellular auxin per-
ception (12, 13, 15, 17, 18, 25). The functions of
ABP1 and TMKs agree with their role in reg-
ulating PIN distribution but also point to un-
explored roles for extracellular auxin in other
pathways (6, 17, 18, 21, 26–28). Therefore, the
discovery of the ABP1-TMK complex under-
lies many exciting prospects of elucidating the
roles of cell surface auxin perception and its
relation with the TIR1/AFB-based nuclear auxin
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Acknowledgments: We are grateful to X. Chen, N. Raikhel,
V. Gonehal, and members of the Yang laboratory for
their stimulating discussion and critical comments on this
manuscript. This work is supported by grants from the U.S.
National Institute of General Medical Sciences to Z.Y.
(GM081451) and A.M.J. (GM065989), the European Research
Council (project ERC-2011-StG-20101109-PSDP) and
CEITEC – Central European Institute of Technology (CZ.1.05/
1.1.00/02.0068) to J.F., and University of Wisconsin HATCH
support to S.E.P. Requests for tmk mutant seeds described in
this work must be sent to N.D. ( firstname.lastname@example.org).
Predoctoral fellowship from the Agency for Innovation by
Science and Technology (IWT) to H.R.
Materials and Methods
Figs. S1 to S7
26 August 2013; accepted 27 January 2014
Fig. 3. Auxin promotes the association of TMK1 with ABP1 in Arabidopsis. (A and B) The association of ABP1 with TMK1-GFP in Arabidopsis leaves was determined by coimmunoprecipitation (Co-IP)
assay. Plasma membrane–localized BRI1-GFP was used as a negative control. The protein complex from leaf
protoplasts treated with different concentrations of auxin (NAA and IAA) was immunoprecipitated by GFP
antibody (A) or ABP1 antibody (B). ABP1 was detected in the TMK1-GFP complex in an auxin-dependent
manner (A). TMK1-GFP was detected in the ABP1 complex, also in an auxin-dependent manner (B). Input
ABP1 indicates the total amount of ABP1 in protein samples before coimmunoprecipitation. (C) A weak
association of TMK1-GFP with an ABP1-5 mutant protein in the abp1-5;TMK1-GFP mutant was not induced
by auxin addition. The signal shown here (B) was obtained by extended exposure, compared with that
shown in (A) (see fig. S6, A and B).
Fig. 4. Auxin promotes the interaction of ABP1 with the extracellular domain of TMK1. (A) The
association of ABP1 with TMK1 or EX-TMK1 was analyzed by coimmunoprecipitation in tobacco leaves that
transiently expressed ABP1 and EX-TMK1 tagged with HPB. Streptavidin-coated magnetic beads were used
to immunoprecipitate TMK1-HPB or EXTMK1-HPB protein complexes, which were immunoblotted with the
ABP1 antibody. The same assay was carried out for ABP1-5 and EX-TMK1 (B).