NMR SOLUTION STRUCTURE OF A CYTOPLASMIC SURFACE LOOP OF THE HUMAN RED-CELL ANION TRANSPORTER, BAND-3

The membrane domain of the human red cell anion transport protein, ban d 3, is too large to be studied by solution nuclear magnetic resonance spectroscopy (NMR), and its amphiphilic nature requires the use of de tergents for solubilization, An alternative approach is to divide the protein into smaller (trans-membrane or surface loop) domains for NMR study. We report the structure of a 46-residue synthetic peptide that corresponds to the cytoplasmic surface loop connecting the putative 12 th and 13th transmembrane spans (residues 796-841) in the 14 span mode l of band 3. This peptide was shown by circular dichroism (CD) to be 3 8% helical in 30% trifluoroacetic acid. Two regions of helix (one clos e to the N-terminus of the peptide and one close to the C-terminus of the peptide) were identified by NMR. Long-range nuclear Overhauser eff ect (NOE) cross-peaks showed the two helices to be in near proximity. The helices were separated by a proline-rich loop that exhibited local order but was mobile with respect to the rest of the peptide, We disc uss how the NMR structure of this loop fits the current models of band 3 structure and topology and the results of recent mutagenesis experi ments. A cyclic version of this peptide was synthesized and studied by CD, but NMR studies were not possible due to the low solubility of th is peptide.