A distinct seven-residue trigger sequence is indispensable for proper coiled-coil formation of the human macrophage scavenger receptor oligomerization domain

We have recently identified a distinct 13-residue sequence pattern that occurs with limited sequence variations in many two-stranded coiled coils but not in trimers, tetramers, or pentamers. This coiled-coil trigger pattern was demonstrated to be indispensable for the assembly of the oligomerization domain of the actin-bundling protein cortexillin I from Dictyostelium discoideum and the leucine zipper domain of the yeast transcriptional activator GCN4. With the aim to extend our knowledge on trigger sequences we have investigated the human macrophage scavenger receptor type A oligomerization domain as a representative of three-stranded coiled coils. We prepared a variety of recombinant N- and C-terminal deletion mutants from the full-length oligomerization domain by heterologous gene expression in Escherichia coli and assessed their ability to form trimeric coiled-coil structures by circular dichroism spectroscopy and analytical ultracentrifugation. Deletion mapping identified a distinct seven-residue sequence that was absolutely required for proper coiled-coil formation, supporting our previous results that heptad repeats alone are not sufficient for oligomerization. The finding that all fragments containing this particular sequence exhibited similar thermal stabilities indicates primarily a stabilizing function of the coiled-coil trigger. Based on sequence similarity, we suggest that functionally related sites are present in other three-stranded coiled-coil proteins.