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Figure 1.
Structural order in the poly-Ig from I-band titin. (A)
Crystal structure of I65–I70. β-sheets are color coded to
emphasize domain torsions. The FG β-hairpin, which claps
against the Ig–Ig transition motif EPP, is colored black. (B)
Modular composition of the I-band of soleus titin from rabbit
(N2A and N2B elements are omitted). Ig domains are represented
as boxes, where orange indicates Ig tightly connected and blue
represents Ig containing a C-terminal three-residue linker.
Annotations refer to conserved features at the Ig–Ig
interfaces, where (i) an FG β-hairpin containing an NxxG
sequence is marked by red asterisks, (ii) interdomain EPP motifs
in green are listed vertically under each domain (Ig exhibiting
a natural E-to-A mutation in this motif are colored salmon), and
(iii) the conserved S/T residue in the BC loop is shown in red.
These features are characteristic of the skeletal but not of the
constitutive Ig tandems. Super-repeats of 6 or 10 Ig are
indicated by capped bars. Domains with previously known
structure are marked with a thick bar. (C) Frontal (Left) and
lateral (Right) views of a predicted model of the complete
skeletal Ig-tandem in one of its putative slack conformations in
solution as calculated from linker arrangements in I65–I70.
(D) Model in extended conformation (C and D are color coded as
in B). (E) EM images (70 × 70 nm^2) of
glycerol-sprayed/rotary shadowed I39–I57 accompanied by its
corresponding model. The model (fragment indicated by arrows in
C) has been oriented to match the micrographs, but no other
manipulation has been applied (the 3D conformation of the
I39–I57 model can be visualized in SI Movie 2).
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