Table I. Structural and sequence comparisons of short-chain helical cytokines.
| SCF | M-CSF | IL-4 | GM-CSF | IL-2 | IL-5 | |
|---|---|---|---|---|---|---|
| SCF | 14.1 (13.0) | 12.7 (12.3) | 12.5 (23.5) | 18.8 (16.4) | 6.7 (21.1) | |
| M-CSF | 64 (1.76) | 14.8 (18.9) | 13.8 (18.3) | 17.5 (17.1) | 10.5 (18.6) | |
| IL-4 | 63 (1.58) | 54 (1.82) | 26.6 (25.0) | 14.5 (22.2) | 18.9 (18.9) | |
| GM-CSF | 48 (1.63) | 58 (1.81) | 64 (1.56) | 9.8 (26.0) | 20.4 (14.7) | |
| IL-2 | 48 (1.70) | 57 (1.58) | 69 (1.33) | 61 (1.48) | 14.5 (22.2) | |
| IL-5 | 45 (1.70) | 38 (1.72) | 53 (1.32) | 49 (1.33) | 62 (1.37) |
Structural comparisons and sequence comparisons between the short-chain helical cytokines are given in the lower left and upper right triangles, respectively. Structural comparisons are given as the maximum number of equivalent α-carbon atoms between two short-chain helical cytokines, and the r.m.s. deviation (Å) (in parentheses). Sequence comparisons are given as the percentage of sequence identity from sequence alignment based on structural superimposition, and that based on the sequence alignment from the BESTFIT program of the GCG package (in parentheses). The latter alignment is based only on maximizing the percentage of identity, similarities and length of the matching sequences, and the sequences submitted to the BESTFIT program were restricted within the region as defined in the PDB files, including the disordered residues. With the advantage of the relatively large number of independent data points (15 pairs), we analyzed the correlation between sequence identity and structural deviation. Without any restriction of structural alignment, the correlation coefficient (C) between structural deviation and sequence identity is –0.21 and the Student’s t probability (P) is 0.44, suggesting little correlation between a specific sequence and the tertiary fold. With the restriction of structural alignment, however, C is –0.30 and P is 0.28, indicating that the structure-based sequence identity and structural deviation are weakly connected (as also observed in another highly diverged protein family, hemoglobin; Aronson et al., 1994).