Secretion of alpha-1-proteinase inhibitor requires an almost full length molecule

In the human disease alpha-1-proteinase inhibitor deficiency, some variants of human alpha-1-proteinase inhibitor are not secreted. These secretory variants contain frameshift mutations leading to products with normal amino acid sequences to the points of the mutations followed by short, aberrant C-terminal sequences and then premature termination (Nukiwa, T., Takahashi, H., Brantly, M., Courtney, M., and Crystal, R. (1987) J. Biol. Chem. 262, 11999-12004; Sifers, R. N., Brashears-Macatee, S., Kidd, V. J., Muensch, H., and Woo, S. L. C. (1988) J. Biol. Chem. 263, 7330-7335; Curiel, D., Brantly, M., Curiel, E., Stier, L., and Crystal, R. G. (1989) J. Clin. Invest. 83, 1144-1152). To examine possible causes for lack of secretion of these null variants, we have altered the alpha-1-proteinase inhibitor cDNA to encode a series of abbreviated forms of this protein that retain authentic sequences to the points of truncation. Examination of the fates of these shortened proteins in transiently transfected Cos 1 cells indicates that the aberrant C-terminal sequences in the naturally occurring variants are not responsible for their lack of secretion and show that truncation prior to Pro391 prevents movement from the endoplasmic reticulum to the Golgi apparatus and therefore secretion. These truncated forms of alpha-1-proteinase inhibitor do not form inclusion bodies in the endoplasmic reticulum, rather they are degraded, probably by the pre-Golgi pathway. Our results support the idea that a sequence of at least 391 of the normal 394 residues is essential for the secretion of alpha-1-proteinase inhibitor and suggest that residue 391 plays an especially important role, perhaps in allowing or directing proper folding or as part of a transport signal, in the secretion of this protein.