Table 2. Interhelical residue pairings at XLAGLIDADG positions 4 and 8.
| G/A pairsa | Position 4 (N-terminal) | Position 8 (C-terminal) | ||||
|---|---|---|---|---|---|---|
| P1↔P2 | Countb | Expc(%) | Obsd(%) | Count | Exp (%) | Obs (%) |
| A↔A | 1 | 0.6 | 0.8 | 7 | 12.7 | 5.3 |
| A↔G | 5 | 5.4 | 3.8 | 23 | 7.9 | 17.6 |
| G↔A | 9 | 8.1 | 6.9 | 52 | 29.5 | 39.7 |
| G↔G | 106 | 77.7 | 80.9 | 7 | 18.4 | 5.3 |
| G↔S | 1 | 0.7 | 0.8 | 15 | 7.8 | 11.5 |
| S↔G | 2 | 2.0 | 1.5 | 10 | 3.3 | 7.6 |
| Othere | 7 | 5.5 | 5.3 | 17 | 20.4 | 13.0 |
| χ2 = 0.59 (10.8, p = 0.001) | χ2 = 37.4 (10.8, p = 0.001) | |||||
aInterhelical residue pairings corresponding to the GxxxG motif; P1 and P2 data analyzed separately.
bTotal of 131 LAGLIDADG two-helix bundles.
cExpected frequency (%) derived from the product of the probability for each residue at the given position.
dObserved frequency (%).
eResidue pairing outliers that did not occur as both P11↔P22 and P12↔P21; for position 4, the following combinations were omitted: A with S or T, G with T, S with S or T, and T with T; for position 8, this included the position 4 omissions along with any combination having a D; see text for details.