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The N-end rule relates the in vivo half-life of a protein to the identity of its N-terminal residue. N-terminal asparagine and glutamine are tertiary destabilizing residues, in that they are enzymatically deamidated to yield secondary destabilizing residues aspartate and glutamate, which are conjugated to arginine, a primary destabilizing residue. N-terminal arginine of a substrate protein is bound by the Ubr1-encoded E3␣, the E3 component of the ubiquitin-proteasome-dependent N-end rule pathway. We describe the construction and analysis of mouse strains lacking the asparagine-specific N-terminal amidase (Nt N -amidase), encoded by the Ntan1 gene. In wild-type embryos, Ntan1 was strongly expressed in the branchial arches and in the tail and limb buds. The Ntan1 ؊/؊ mouse strains lacked the Nt N -amidase activity but retained glutamine-specific Nt Q -amidase, indicating that the two enzymes are encoded by different genes. Among the normally short-lived N-end rule substrates, only those bearing N-terminal asparagine became long-lived in Ntan1 ؊/؊ fibroblasts. The Ntan1 ؊/؊ mice were fertile and outwardly normal but differed from their congenic wild-type counterparts in spontaneous activity, spatial memory, and a socially conditioned exploratory phenotype that has not been previously described with other mouse strains.

Context discrimination and time course studies of contextual fear conditioning revealed strain differences between C57BL/6J (B6) and DBA/2J (D2) mice. Both strains discriminated contexts, but D2 mice exhibited less freezing in a shock-paired context. The strains did not differ immediately, or at 1 and 3 hr after contextual fear conditioning training. D2 mice showed less freezing at 15 min, 30 min, and 24 hr after training. B6 mice exhibited exaggerated generalized freezing and poor discrimination between the context and altered context 7-30 days after training. The acoustic startle response in B6 mice was also enhanced at 14 days after training. D2 mice did not show this pattern of generalized freezing. B6, but not D2, mice retained contextual memories for at least 60 days.

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Nicotine-stimulated 86 Rb ϩ efflux and [ 3 H]cytisine binding, both of which seem to measure the nicotinic acetylcholine receptor, composed of ␣4 and ␤2 subunits, were assessed in eight brain regions obtained from 14 inbred mouse strains. The potential role of a single nucleotide polymorphism (SNP) in the nicotinic receptor ␣4 subunit gene (Chrna4) on nicotinic receptor binding and function in mice was also evaluated. This SNP leads to an alanine-to-threonine variation at amino acid position 529 of the nascent ␣4 subunit polypeptide. Both nicotine-stimulated 86 Rb ϩ efflux and [ 3 H]cytisine binding were found to vary across brain regions and among mouse strains. Variability in nicotinestimulated 86 Rb ϩ efflux was positively correlated (r Ͼ 0.9) within each strain with the number of [ 3 H]cytisine binding sites.

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alpha4beta2-containing nAChRs may play important roles in modulating the effects of both ethanol and nicotine on the acoustic startle response. We suggest that nAChR subunit genes should be evaluated as potential contributors to both alcoholism and tobacco abuse.