Background: The seven-residue heptad repeat is the accepted hallmark o
f coiled coils. In extended filamentous proteins, however, contiguous
patterns of heptads are often disrupted by 'skips' and 'stammers'. The
structural consequences and roles of these digressions are not unders
tood. Results: In a cytoskeleton protein from Giardia lamblia, heptads
flank eleven-residue units (hendecads) to give a 7-11-7 motif that do
minates the sequence. Synthetic peptides made to the consensus sequenc
e of this motif fold in solution to fully helical, parallel dimers. Bo
th the sequence pattern and these experimental data are consistent wit
h the coiled-coil model, We note that breaks in other extended coiled
coils can also be reconciled by hendecad insertions, Conclusions: The
heptad paradigm for the coiled coil must be expanded to include hendec
ads. As different combinations of heptads and hendecads will give diff
erent overall sequence motifs, we propose that these provide a mechani
sm to promote cognate protein pairings during the folding of extended
coiled coils in the cell.