Autoprocessing of HIV-1 protease is tightly coupled to protein folding
John M. Louis, G. Marius Clore
& Angela M. Gronenborn
Laboratory of Chemical Physics, Building 5, National Institute of
Diabetes and Digestive and Kidney Diseases, National Institutes of Health
, Bethesda, Maryland 20892, USA
.
Correspondence should be addressed to Angela M. Gronenborn gronenborn@nih.gov
In the Gag-Pol polyprotein of HIV-1, the 99-amino acid protease is flanked
at its N-terminus by a transframe region (TFR) composed of the transframe
octapeptide (TFP) and 48 amino acids of the p6pol, separated
by a protease cleavage site. The intact precursor (TFP-p6pol-PR)
has very low dimer stability relative to that of the mature enzyme and exhibits
negligible levels of stable tertiary structure. Thus, the TFR functions by
destabilizing the native structure, unlike proregions found in zymogen forms
of monomeric aspartic proteases. Cleavage at the p6pol-PR site
to release a free N-terminus of protease is concomitant with the appearance
of enzymatic activity and formation of a stable tertiary structure that is
characteristic of the mature protease as demonstrated by nuclear magnetic
resonance. The release of the mature protease from the precursor can either
occur in two steps at pH values of 4 to 6 or in a single step above pH 6.
The mature protease forms a dimer through a four-stranded -sheet at
the interface. Residues 1−4 of the mature protease from each subunit
constitute the outer strands of the -sheet, and are essential for maintaining
the stability of the free protease but are not a prerequisite for the formation
of tertiary structure and catalytic activity. Our experimental results provide
the basis for the model proposed here for the regulation of the HIV-1 protease
in the viral replication cycle.
-sheet at
the interface. Residues 1−4 of the mature protease from each subunit
constitute the outer strands of the 
