Characterization of the gene and protein of the α1-antitrypsin 'deficiency' allele M(procida)
alpha 1-Antitrypsin Deficiency
The "deficiency" group of alpha 1-antitrypsin (alpha 1AT) alleles is characterized by alpha 1AT genes that code for alpha 1AT present in serum but in amounts insufficient to protect the lower respiratory tract from progressive destruction by its burden of neutrophil elastase. Mprocida, a rare alpha 1AT allele associated with alpha 1AT serum levels less than 10 mg/dl (normal 150-350 mg/dl), codes for an alpha 1AT molecule that focuses on immobilized pH gradient isoelectric gels slightly cathodal to the common normal M1 (Val213) protein. On a per molecule basis, Mprocida has a mildly reduced function as an inhibitor, with an association rate constant for human neutrophil elastase of 7.0 +/- 0.1 x 10(6) M-1 s-1 (normal M1 (Val213) 9.3 +/- 0.8 x 10(6), p less than 0.01). The Mprocida molecule behaves normally in vivo with a half-life similar to normal M1 alpha 1AT molecules. Restriction endonuclease mapping demonstrates that the cloned Mprocida gene was grossly intact. Sequencing of all the exons, exon-intron junctions, and the major promoter region demonstrated Mprocida to be identical to the M1 (Val213) gene except for a single base substitution in exon II coding for amino acid 41 of the mature protein (M1 (Val213) Leu41 CTG----Mprocida Pro41 CCG). Usefully, the coding sequence of the alpha 1AT residues 40-41 is recognized by the restriction endonuclease PvuII so that using a probe corresponding to this region of exon II, the Mprocida mutation can be rapidly identified by Southern analysis. Evaluation of the crystallographic structure of alpha 1AT suggests the Leu41 to Pro41 mutation may disrupt alpha-helix A in the region of Pro21-Ser45, suggesting the possibility that the alpha 1AT Mprocida molecule is unstable and degraded intracellularly prior to secretion.