The human immunodeficiency virus type 1 aspartyl protease (HIV-1 PR) is a homodimeric aspartyl endopeptidase that is required for virus replication. HIV-1 PR was shown to act in vitro as acyl-donor and-acceptor for both guinea pig liver transglutaminase (TG, EC 2.3.2.13) and human Factor XIlla. These preliminary evidences suggested that the HIV-1 PR contains at least three TG-reactive glutaminyl and one lysyl residues. We report here that the incubation of HIV-1 PR with TG increases its catalytic activity. This increase is dependent upon the time of incubation, the concentration of TG and the presence of Ca2+. Identification of epsilon-(gamma-glutamyl)lysine in the proteolytic digest of the TG-modified HIV-1 PR suggested intramolecular covalent cross-linking of this protease which may promote a non-covalent dimerization and subsequent activation of this enzyme via a conformational change. This hypothesis is supported by the observation that the TG-catalyzed activation of HIV-1 PR was completely abolished by spermidine (SPD) which acts as a competitive inhibitor of epsilon-(gamma-glutamyl)lysine formation. Indeed, in the presence of 1 mM SPD the formation of the isopeptide was decreased of about 80%. The main products of the TG-catalyzed modification of HIV-1 PR in the presence of SPD were N-1-mono(gamma-glutamyl)SPD and N-8-mono(gamma-glutamyl)SPD. Negligible amount of N-1,N-8-bis(gamma-glutamyl)SPD were found. The significance of these results is discussed with respect to the activation of the protease by post-translational modification and design of potential inhibitors. (C) 2010 Elsevier Inc. All rights reserved.