Using MTBindingSim as a Tool for Experimental Planning and Interpretation - Methods in Cell Biology - page 371

Next, the behavior of models in which Tau binds to MT-bound Tau can be exam-
ined. Figure 23.2 C shows the amount of Tau that binds to 1 MMTs using the “MAPs

bind MT-bound MAPs” model. Three different scenarios are shown: the MAP-MT-
binding affinity is weaker than the MAP-MAP binding, the MAP-MT- and the
MAP-MAP-binding affinities are equal, and theMAP-MT-binding affinity is stronger
than the MAP-MAP affinity. Figure 23.2 C shows that, in any scenario in which the
one MAP is allowed to bind each MT-bound MAP, all curves saturate at 2

m

Mbound

m

MAP, unlike what happens with the data that are shown in Fig. 23.2 A.
To test the hypothesis that the data in Fig. 23.2 A can be explained by adding an
additional layer of Tau-Tau interaction, the “Two MAPs bind MT-bound MAPs”
model was used with the same binding affinities as those set in Fig. 23.2 C
( Fig. 23.2 D). The results of Fig. 23.2 D are similar to those shown in Fig. 23.2 C in that
the curves saturate; the amount of MAP bound simply saturates at 3 M instead of

m

2 M. More importantly, Fig. 23.2 C and D demonstrates that, for any small number

of Tau-Tau interactions at theMT surface, therewill be a clear upper bound limit in the
amount of Tau bound. This behavior is in contrast to the continual rise in the amount of
Tau bound seen in the experimental data of Fig. 23.2 A.
These observations suggest that a model based on Tau-MT interactions and finite
Tau-Tau oligomerization is not able to explain the data observed in Fig. 23.2 A.
There must be another interaction occurring in solution between the Tau and MT
proteins that has not yet been accounted for by the model. Duan and Goodson have
recently used fluorescence microscopy to provide evidence that this “additional pro-
cess” is formation of Tau-only filaments that are induced by the presence of MTs.
The presence of these filaments confounds the MT cosedimentation assays by re-
moving free Tau from solution (causing an apparent reduction of Tau-MT affinity)
and by sedimenting independently of MTs (increasing the apparent affinity) ( Duan &
Goodson, 2012 ).
This work shows the value of MTBindingSim as a tool for interpreting binding
data. Using MTBindingSim demonstrated that the observations of supersaturation
in the Tau-MT experiments could not be due to Tau oligomerization at the MT surface
as had previously been suggested. This conclusion opened up consideration to other
possible explanations for the observed Tau-MT behavior and ultimately led to the mi-
croscopy studies that revealed the cause of the puzzling binding data. Without the aid
of MTBindingSim, it would have been more difficult to confidently reject the hypoth-
esis of Tau oligomerization on the MT surface and then move on to other possibilities.

m

23.5 USING MTBindingSim EXAMPLE 2: DOES HUMAN EB1
BIND TO THE MT SEAM OR LATTICE?
One class of MAPs of particular interest is the MT plus-end tracking proteins
( þ TIPs). One þ TIP that has been of particular interest is EB1, which is considered
the “core” of the þ TIP network ( Akhmanova & Steinmetz, 2008 ). There are conflict-
ing reports in the published literature about the nature of EB1-MT-binding interac-
tions and whether EB1 has a preference for the MT seam. The MT seam is a