Double Dissociation of Amygdala and Hippocampal Contributions to Trace and Delay Fear Conditioning

A key finding in studies of the neurobiology of learning memory is that the amygdala is critically involved in Pavlovian fear conditioning. This is well established in delay-cued and contextual fear conditioning; however, surprisingly little is known of the role of the amygdala in trace conditioning. Trace fear conditioning, in which the CS and US are separated in time by a trace interval, requires the hippocampus and prefrontal cortex. It is possible that recruitment of cortical structures by trace conditioning alters the role of the amygdala compared to delay fear conditioning, where the CS and US overlap. To investigate this, we inactivated the amygdala of male C57BL/6 mice with GABA A agonist muscimol prior to 2-pairing trace or delay fear conditioning. Amygdala inactivation produced deficits in contextual and delay conditioning, but had no effect on trace conditioning. As controls, we demonstrate that dorsal hippocampal inactivation produced deficits in trace and contextual, but not delay fear conditioning. Further, pre- and post-training amygdala inactivation disrupted the contextual but the not cued component of trace conditioning, as did muscimol infusion prior to 1- or 4-pairing trace conditioning. These findings demonstrate that insertion of a temporal gap between the CS and US can generate amygdala-independent fear conditioning. We discuss the implications of this surprising finding for current models of the neural circuitry involved in fear conditioning

Association between an oncogene and an anti-oncogene: the adenovirus E1A proteins bind to the retinoblastoma gene product

One of the cellular targets implicated in the process of transformation by the adenovirus E1A proteins is a 105K cellular protein. Previously, this protein had been shown to form stable protein/protein complexes with the E1A polypeptides but its identity was unknown. Here, we demonstrate that it is the product of the retinoblastoma gene. The interaction between E1A and the retinoblastoma gene product is the first demonstration of a physical link between an oncogene and an anti-oncogene

Role of Arginines in Coenzyme A Binding and Catalysis by the Phosphotransacetylase from Methanosarcina thermophila

Phosphotransacetylase (EC 2.3.1.8) catalyzes the reversible transfer of the acetyl group from acetyl phosphate to coenzyme A (CoA): CH(3)COOPO(3)(2−) + CoASH ⇆ CH(3)COSCoA + HPO(4)(2−). The role of arginine residues was investigated for the phosphotransacetylase from Methanosarcina thermophila. Kinetic analysis of a suite of variants indicated that Arg 87 and Arg 133 interact with the substrate CoA. Arg 87 variants were reduced in the ability to discriminate between CoA and the CoA analog 3′-dephospho-CoA, indicating that Arg 87 forms a salt bridge with the 3′-phosphate of CoA. Arg 133 is postulated to interact with the 5′-phosphate of CoA. Large decreases in k(cat) and k(cat)/K(m) for all of the Arg 87 and Arg 133 variants indicated that these residues are also important, although not essential, for catalysis. Large decreases in k(cat) and k(cat)/K(m) were also observed for the variants in which lysine replaced Arg 87 and Arg 133, suggesting that the bidentate interaction of these residues with CoA or their greater bulk is important for optimal activity. Desulfo-CoA is a strong competitive inhibitor of the enzyme, suggesting that the sulfhydryl group of CoA is important for the optimization of CoA-binding energy but not for tight substrate binding. Chemical modification of the wild-type enzyme by 2,3-butanedione and substrate protection by CoA indicated that at least one reactive arginine is in the active site and is important for activity. The inhibition pattern of the R87Q variant indicated that Arg 87 is modified, which contributes to the inactivation; however, at least one additional active-site arginine is modified leading to enzyme inactivation, albeit at a lower rate