Tripartite motif protein TRIM5α blocks retroviral replication after cell entry, and species-specific differences in its activity are determined by sequence variations within the C-terminal B30.2/ PRYSPRY domain. Here we report a high-resolution structure of a TRIM5α PRYSPRY domain, the PRYSPRY of the rhesus monkey TRI-M5α that potently restricts HIV infection, and identify features involved in its interaction with the HIV capsid. The extensive capsidbinding interface maps on the structurally divergent face of the protein formed by hypervariable loop segments, confirming that TRIM5α evolution is largely determined by its binding specificity. Interactions with the capsid are mediated by flexible variable loops via a mechanism that parallels antigen recognition by IgM antibodies, a similarity that may help explain some of the unusual functional properties of TRIM5α. Distinctive features of this pathogenrecognition interface, such as structural plasticity conferred by the mobile v1 segment and interaction with multiple epitopes, may allow restriction of divergent retroviruses and increase resistance to capsid mutations.

The Cdc25 phosphatases play key roles in cell-cycle progression by activating cyclin-dependent kinases. The latter are absent from neurons that are terminally differentiated in adult brain. However, accumulation of mitotic phosphoepitopes, and re-expression and activation of the M phase regulator, Cdc2/cyclin B, have been described in neurons undergoing degeneration in Alzheimer's disease (AD). To explain this atypical mitotic activation in neurons we investigated the Cdc2-activating Cdc25A phosphatase in human brain. The structural hallmarks of AD neurodegeneration, neurofibrillary tangles and neuritic plaques, were prominently immunolabeled with Cdc25A antibodies. In addition numerous neurons without visible structural alterations were also intensely stained, whereas control brain was very weakly positive. After immunoprecipitation from control and AD tissue, we found that the tyrosine dephosphorylating activity of Cdc25A against exogenous Cdc2 substrate was elevated in AD. Accordingly, Cdc25A from AD tissue displayed increased immunoreactivity with the mitotic phosphoepitope-specific antibody, MPM-2, and co-localized with MPM-2 immunoreactivity in AD neurons. These data suggest that Cdc25A participates in mitotic activation during neurodegeneration. The involvement of Cdc25A in cellular transformation, modulation of the DNA damage checkpoint, and linkage of mitogenic signaling to cell cycle machinery, also implicates one of these cell-cycle pathways in AD pathogenesis.

It is commonly believed that neurons remain in G 0 phase of the cell cycle indefinitely. Cell-cycle re-entry, however, is known to contribute to neuronal apoptosis. Moreover, recent evidence demonstrates the expression of cell-cycle proteins in differentiated neurons under physiological conditions. The functional roles of such expression remain unclear. Since DNA repair is generally attenuated by differentiation in most cell types, the cell-cycle-associated events in postmitotic cells may reflect the need to re-enter the cell cycle to activate DNA repair. We show that cyclin-C-directed, pRb-dependent G 0 exit activates the nonhomologous end joining pathway of DNA repair (NHEJ) in postmitotic neurons. Using RNA interference, we found that abrogation of cyclin-C-mediated exit from G 0 compromised DNA repair but did not initiate apoptosis. Forced G 1 entry combined with prevention of G 1 -S progression triggered NHEJ activation even in the absence of DNA lesions, but did not induce apoptosis in contrast to unrestricted progression through G 1 -S. We conclude that G 0 -G 1 transition is functionally significant for NHEJ repair in postmitotic neurons. These findings reveal the importance of cell-cycle activation for controlling both DNA repair and apoptosis in postmitotic neurons, and underline the particular role of G 1 -S progression in apoptotic signaling, providing new insights into the mechanisms of DNA damage response (DDR) in postmitotic neurons.