Study of the Roles of Proline 391 and a Highly Conserved Sequence in the Carboxyl-terminal Region of Members of the Serpin Family in the Secretion of a1-Proteinase Inhibitor*

Truncation of al-proteinase inhibitor prior to Pros0' prevents its secretion. This residue is the carboxyl terminus of a highly conserved sequence in serpins, suggesting that either Pros0' or the conserved sequence may serve as at least a part of a signal recognized by components of the secretory pathway. To evaluate these possibilities, we have determined the effects of replacement of ProS0' on the secretion of al-proteinase inhibitor and have examined the ability of the 9-residue conserved sequence to mediate secretion. We find that replacement of Pros0' with hydrophobic residues yields variants that are well secreted, but replacement with other classes of amino acids severely restricts secretion. These results show that while al-proteinase inhibitor is secreted most efficiently when proline occupies position 391, Pros0' is not an absolute requirement for its secretion. Our results show that the 9-amino acid conserved sequence found near the carboxyl termini of proteins of the serpin family is not sufficient to direct the secretion of al-proteinase inhibitor. We conclude that mutations affecting residue 391 and other positions in the conserved region lead to structural changes, possibly very minor, that influence the secretion of al-proteinase inhibitor.