Figure 1.

(A) Secondary structure predictions performed with the PSA server (http://bmerc-www.bu.edu/psa; White et al. 1994) indicate that the β_4a-A domain has a mixture of α-helix and β-sheet. (B) Secondary structural characterization by multidimensional NOESY-HSQC and HNHA J coupling experiments. Solid bars for dNN (i, i + 1) and d_{α N} (i, i + 1) represent continuous cross peaks observed in ¹⁵N-edited NOESY-HSQC experiments run with a 50-msec mixing time. Lines for d_{α N} (i, i + 3) and d_{α N} (i, i + 4) represent NOE cross-peaks between residues three and four amino acids away, respectively. The ³J_HNHA coupling constants provide further information on the secondary structure boundaries. These results outline the secondary structural elements present in the β_4a-A domain and correlate with predicted structural elements. (C ) Solution structure of the β_4a-A domain determined with a backbone RMSD of 0.73 A °(Table 1). The β_4a-A domain is independently folded with the first helix (α 1) coordinating two β-strands (β1 and 2), three loops (L1–3), and a C-terminal α-helix (α 2) to create a globular, hydrophobic core. (D) The α 2 helix is involved in packing against the SH3 domain (Opatowsky et al. 2004). Conserved residues are involved in hydrophobic packing (A54, L51, and A47 on the A domain, and V80, F100, W104, and I106 on the SH3 domain) and formation of salt bridges (R53–E97 and K55–E115) between the A and the SH3 domains of β_4a.