Interactions near the conserved tryptophan residues in the βI-14 and βI-15 repeats. The diagrams show space-filling models with the conserved hydrophobic triplets depicted in orange and other interacting residues colored light blue. (A) The 14th repeat contains 2 conserved tryptophan residues, W₁₇₀₅ and W₁₇₇₉. W₁₇₀₅ is in the highly conserved g position of the second heptad of h14A and interacts with the d position residue of the third heptad of h14B (I₁₇₄₂). The semiconserved W₁₇₇₉ is in the a position of the second heptad of h14C and interacts with both W₁₇₀₅ and I₁₇₄₂ as well as with I₁₇₀₆. (B) The 15th repeat lacks both the “invariant” tryptophan in h15A and the semiconserved histidine in h15B. Instead, this repeat contains L₁₈₁₂ and L₁₈₄₈ and the semi-invariant W₁₈₈₅. L₁₈₄₈ interacts with W₁₈₈₅, whereas L₁₈₁₂ interacts with I₁₈₀₉. The core is further stabilized by V₁₈₅₂ and V₁₈₈₁ that interact with I₁₈₀₉ and W₁₈₈₅, respectively. The cores of both βI-14 and βI-15 are thus tightly packed and hydrophobic, without included water. This is unlike the structure reported for βI-9 that also lacks the invariant tryptophan.³⁹