Figure 4: CASP15 Protein assembly subunit interface
agreement with experiment. Bars show the best agreement obtained between
computed and experimental interfaces, using the Interface Contact Score
(ICS). Green bars indicate interfaces in targets where there are
homologous structures for all component subunits and interfaces. Blue
bars are those where there are homologous structures for some subunits
and interfaces, and red those where there is no structural homology.
More than half the interfaces reach a score of 0.8 or higher, and so may
approach experimental accuracy. About a quarter reach a more stringent
criterion of ICS > 90.
In CASP15 a total of 40 protein assembly targets were assessed, ranging
from simple obligate dimers to very large complexes with up to 11
polypeptide chains and up to more than 4000 amino acids. Figure 4 shows
the distribution of interface structure agreement with experiment in
terms of the Interface Contact score (ICS), the F1 measure of contact
agreement introduced earlier. Examination of interfaces with lowest ICS
values ( <0.6) is informative. Three of the eight targets
bearing the poorly modeled interfaces are immune complexes, discussed
below. Four interfaces are from large complexes, which tend to be a
slightly lower accuracy overall, and one is for a viral coat protein
homodimer (T1123). The multiple sequence alignment for the monomer of
this homo-dimeric target is shallow (10 sequences), but the subunit’s
best models are of high accuracy (best GDT_TS of 87), so it is unclear
why this interface has poor agreement with experiment.