Figure 1.

Amino acid frequencies in the β-strands of 16 β-barrel membrane proteins. (A) The frequency of amino acids facing the membrane lipids in the N-terminal (black) and C-terminal (gray) halves of the membrane. Glu and Arg are not found in the N-terminal half. (B) The statistical significance of the N/C bias of lipid-facing amino acids. For each amino acid, we show the random probability of having the observed distribution, or a more biased distribution, for one particular membrane half. A small probability indicates that the likelihood of having the observed bias is improbable. Amino acids with a gray bar above the drawn line have distributions with less than a 5% chance of occurring at random. (C) The frequency of amino acids facing into the β-barrel in the N-terminal (black) and C-terminal (gray) halves of the strands. (D) The statistical significance of the N/C bias of amino acids facing into the β-barrel center. The amino acids are listed in order of decreasing hydrophobicity (Engelman et al. 1986).