The Oct-1 POU homeodomain stabilizes the adenovirus preinitiation complex via a direct interaction with the priming protein and is displaced when the replication fork passes.

Initiation of adenovirus DNA replication is strongly enhanced by two cellular transcription factors, NFI and Oct-1, which bind to the auxiliary origin and tether the viral precursor terminal protein-DNA polymerase (pTP.pol) complex to the core origin. NFI acts through a direct contact with the DNA polymerase, but the mode of action of Oct 1 is unknown. Employing glutathione S-transferase-POU pull-down assays and protein affinity chromatography, we have established that the POU domain contacts pTP rather than pol. The POU homeodomain is responsible for this interaction. The protein-protein contacts lead to increased binding of pTP-pol to the core origin, which is caused by a reduced off-rate. The enhanced formation of a pTP.pol.POU complex on the origin correlates with stimulation of replication. Using an immobilized replication system, we have studied the kinetics of dissociation of the Oct-1 POU domain during replication. In contrast to NFI, which dissociates very early in initiation, Oct-1 dissociates only when the binding site is rendered single-stranded upon translocation of the replication fork. Our data indicate that NFI and Oct-1 enhance initiation synergistically by touching different targets in the preinitiation complex and dissociate independently after initiation.

Linker length and composition influence the flexibility of Oct-1 DNA binding.

POU domain transcription factors have two separate helix-turn-helix DNA-binding subdomains, the POU homeodomain (POUhd) and the POU-specific domain (POUs). Each subdomain recognizes a specific subsite of 4 or 5 bp in the octamer recognition sequence. The Oct-1 POU subdomains are connected by a 23 amino acid unstructured linker region. To investigate the requirements for the linker and its role in DNA recognition, we constructed POU domains in which the subdomains are connected with linkers varying in length between 2 and 37 amino acids. Binding to the natural octamer site required a minimal linker length of between 10 and 14 amino acids. A POU domain with an eight amino acid linker, however, had a high affinity for a site in which the POUs recognition sequence was inverted. Computer modelling shows that inversion of the POUs subdomain shortens the distance between the subdomains sufficiently to enable an eight amino acid linker to bridge the distance. DNase I footprinting as well as mutation of the POUs-binding site confirms the inverted orientation of the POUs domain. Switching of the POUs and POUhd subdomains and separation by 3 bp leads to a large distance which could only be bridged effectively by a long 37 amino acid linker. In addition to linker length, mutation of a conserved glutamate residue in the linker affected binding. As shown by surface plasmon resonance measurements, this was caused by a decrease in the on-rate. Our data indicate that there are both length and sequence requirements in the linker region which allow flexibility leading to selective binding to differently spaced and oriented subsites.

Distribution of glutamate transporters in the hippocampus of patients with pharmaco-resistant temporal lobe epilepsy.

In patients suffering from temporal lobe epilepsy (TLE), increased extracellular glutamate levels in the epileptogenic hippocampus both during and after clinical seizures have been reported. These increased glutamate levels could be the result of malfunctioning and/or downregulation of glutamate transporters (also known as EAATs; excitatory amino acid transporters). In this study, the distribution of protein and mRNA of EAAT subtypes was examined in the hippocampus of TLE patients with hippocampal sclerosis (HS group) and without hippocampal sclerosis (non-HS group), and in autopsy controls without neurological disorders. EAAT protein localization was studied by immunohistochemistry on paraffin sections using specific poly- and monoclonal antibodies against the glial glutamate transporters EAAT1 and EAAT2 and the neuronal glutamate transporter EAAT3. Antibody specificity was shown by immunoblotting. In the HS group, a small decrease in EAAT1-immunoreactivity (IR) was observed in CA4 and in the polymorphic and supragranular layer of the dentate gyrus, compared with the control group. The strongest changes were found for EAAT2 levels. In the non-HS group, increased EAAT2-IR was detected in the CA1 and CA2 field, compared with non-epileptic controls. EAAT2-IR was decreased in the HS compared with the non-HS group. Fewer EAAT3-positive cells were found in the HS group than in the non-HS and control group. In both TLE groups, increased EAAT3 levels were observed in individual neurones. In the HS group, the percentage of EAAT3-IR neurones was increased in CA2 and in the granule cell layer of the dentate gyrus. Radioactive in situ hybridization for EAAT1-3 confirmed our immunohistochemical results. Non-radioactive in situ hybridization showed that not only astrocytes, but also neurones express EAAT2 mRNA. Taken together, differences in both mRNA and protein levels of glutamate transporter subtypes were found in specific regions in the TLE hippocampus, with most severe changes found for EAAT2 and EAAT3 levels. The results indicate an upregulation of EAAT2 protein expression in CA1 and CA2 in neurones in the non-HS group. This is in line with decreased EAAT2 protein levels in the HS group, since these hippocampi are characterized by severe neuronal cell loss. The functional consequences (glutamate transport capacity) of the reported changes in EAAT2 and EAAT3 remain to be determined.