A THERMOSTABLE MUTATION LOCATED AT THE HYDROPHOBIC CORE OF ALPHA(1)-ANTITRYPSIN SUPPRESSES THE FOLDING DEFECT OF THE Z-TYPE VARIANT

A thermostable mutation, F51L, at the hydrophobic core of human alpha( 1)-antitrypsin (alpha(1)AT) increased the conformational stability of the molecule by decreasing the unfolding rate significantly without al tering the refolding rate, The mutation specifically influenced the tr ansition between the native state and a compact intermediate, which re tained similar to 70% of the far-UV CD signal, but which had most of t he fluorescence signal already de quenched. The mutant alpha(1)AT prot ein was more resistant than the wild-type protein to the insertion of the tetradecapeptide mimicking the sequence of the reactive center loo p, indicating that the mutation increases the closing of the central b eta-sheet, the A-sheet, in the native state. The F51L mutation enhance d the folding efficiency of the Z-type (E342K) genetic variation, whic h causes aggregation of the molecule in the liver. It has been shown p reviously that the aggregation of the Z protein occurs via loop sheet polymerization, in which the reactive center loop of one molecule is i nserted into the opening of the A-sheet of another molecule. Our resul ts strongly suggest that the hydrophobic core of alpha(1)AT regulates the opening-closing of the A-sheet and that certain genetic variations that cause opening of the A-sheet can be corrected by inserting an ad ditional stable mutation into the hydrophobic core.