Behavior of mice lacking components of an intracellular proteolytic pathway

Protein synthesis is generally regarded as a prerequisite for some types of learning and memory processes (Bourtchouladze et al., 1998). Gene targeting technology has made it possible to investigate, for the first time, what role protein degradation plays in these same processes. ^ Features of proteins which signal metabolic instability upon some or all their peptide bonds are called degrons (Varshavsky, 1991). One signaling pathway, just prior to and recognized by the ubiquitin-mediated proteolytic system, is called the Wend rule pathway (Byrd et al., 1998). The Wend rule relates the in vivo half-life of a protein to the identity of its N-terminal amino acid (Bachmair et al., 1986). ^ The Wend rule pathway is the enzymatic machinery that carries out the rate specific degradation of proteins described by the N-end rule. The N-end rule pathway consists primarily of three enzymes. These include, N-terminal amidases, aminoacyl-transfer RNA-protein transferase (R-transferase), and N-recognin (or E3) (Varshavsky, 1996). The asparagine-specific N-terminal amidase is necessary for the chemical modification (deamidation) of asparagine bearing proteins (Grigoryev et al., 1996). This allows them to be further modified (arginlyated) by an R-transferase prior to entering the ubiquitin-mediated pathway (Ingoglia et al., 1983). All proteins\u27 N-termini must be recognized by Nα-recognin before they can be degraded. ^ The current research focused primarily upon the behavioral characterization of two mouse knockouts. The first had the NTAN1 gene, which codes for the asparagine-specific N-terminal amidase removed, and the other had the UBR1 gene, which codes for E3α removed. The removal of the asparagine-specific N-terminal amidase results in proteins bearing asparagine to become metabolically stabilized indefinitely, whereas removal of E3α results in the stabilization of 13 amino acid residues, including asparagine. ^ Because several important physiological substrates, known for their involvement in learning and memory, have been shown to be degraded via the N-end rule and ubiquitin-mediated pathways (e.g., a cAMP repressor, the Gp-protein α subunit, protein kinase C, and c-fos) (Hochstrasser & Kornitzer, 1998; Lu et al., 1998; Folco & Koren, 1997; Madura & Varshavsky, 1994), several cognitive variables were investigated.