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Abstract. The 17-juxtamembrane cytoplasmic residues of the polymeric immunoglobulin receptor contain an autonomous basolateral targeting signal that does not mediate rapid endocytosis (Casanova, J. E., G. Apodaca, and K. E. . Alanine-scanning mutagenesis identifies three residues in this region, His656, Arg657, and Va1660, that are most essential for basolateral sorting and two residues, Arg655 and Tyr668, that play a lesser role in this process. Progressive truncations suggested that Ser664 and Ile665 might also play a role in basolateral sorting. However, mutation of these residues to Ala or internal deletions of these residues did not affect basolateral sorting, indicating that these residues are probably not required for basolateral sorting. Twodimensional NMR spectroscopy of a peptide corresponding to the 17-mer signal indicates that the sequence Arg658-Asn-Val-Asp661 has a propensity to adopt a/if-turn in solution. Residues COOH-terminal to the B-turn (Arg662 to Arg669) seem to take up a nascent helix structure in solution. Substitution of Va1660 with Ala destabilizes the turn, while mutation of Arg657 to Ala does not appear to affect the turn structure. Neither mutation detectably altered the stability of the nascent helix in the COOH-terminal portion of the peptide.p OLARIZED epithelial cells have two plasma membrane domains: the apical domain facing the external environment, and the basolateral domain facing the internal milieu. These domains display striking differences in protein and lipid composition (Caplan and Matlin, 1989;

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As part of a conformational study of the pathway of unfolding and refolding of bovine pancreatic trypsin inhibitor that accompanies breakage and formation of its three disulfide bonds, circular dichroism spectra have been measured for several limiting conformational states: native and refolded, with the three correct disulfide bonds; the (30--51, 5--55) two-disulfide species trapped during unfolding and refolding, which have a stable nativelike conformation; the fully reduced protein, with no disulfide bonds. Refolded protein with the three correct disulfide bonds has been found to be slightly different from the native protein; this conformational difference could be removed by gently heating the refolded protein. The same difference appears to be present between the two-disulfide intermediates, lacking the 14--38 disulfide bond, produced during unfolding and refolding. The conformational difference appear to be introduced at an early stage of refolding. The fully reduced protein, with no disulfides, exists as a flexible polypeptide chain with no detectable fixed conformation. The near-ultraviolet portions of the spectra are resolved into probable contributions by tyrosine, disulfide, and phenylalanine side-chain electronic transitions. The probable contributions to the native protein spectrum by tyrosines were also elucidated by observing the spectral shifts caused by their ionization at pH 12.5, where the folded conformation is maintained. The rotational strengths of the isolated transitions provide a measure of conformational flexibilities for the chromophores. Resolution of the far-ultraviolet spectrum of the native protein into contributions of its known secondary structures was not successful.