Zinc finger protein ZFP57 requires its co-factor to recruit DNA methyltransferases and maintains DNA methylation imprint in embryonic stem cells via its transcriptional repression domain

Previously, we discovered that ZFP57 is a maternal-zygotic effect gene, and it maintains DNA methylation genomic imprint at multiple imprinted regions in mouse embryos. Despite these findings, it remains elusive how DNA methyltransferases are targeted to the imprinting control regions to initiate and maintain DNA methylation imprint. To gain insights into these essential processes in genomic imprinting, we examined how ZFP57 maintains genomic DNA methylation imprint in mouse embryonic stem (ES) cells. Here we demonstrate that the loss of ZFP57 in mouse ES cells led to a complete loss of genomic DNA methylation imprint at multiple imprinted regions, similar to its role in mouse embryos. However, reintroduction of ZFP57 into Zfp57-null ES cells did not result in reacquisition of DNA methylation imprint, suggesting that the memory for genomic imprinting had been lost or altered in Zfp57-null ES cells in culture. Interestingly, ZFP57 and DNA methyltransferases could form complexes in the presence of KAP1/TRIM28/TIF1β when co-expressed in COS cells. We also found that the wild-type exogenous ZFP57 but not the mutant ZFP57 lacking the KRAB box that interacts with its co-factor KAP1/TRIM28/TIF1β could substitute for the endogenous ZFP57 in maintaining the DNA methylation imprint in ES cells. These results suggest that ZFP57 may recruit DNA methyltransferases to its target regions to maintain DNA methylation imprint, and this interaction is likely facilitated by KAP1/TRIM28/TIF1β.     

The TRIM5alpha B-box 2 domain promotes cooperative binding to the retroviral capsid by mediating higher-order self-association

The retroviral restriction factor, TRIM5α, blocks infection of a spectrum of retroviruses soon after virus entry into the cell. TRIM5α consists of RING, B-box 2, coiled-coil, and B30.2(SPRY) domains. The B-box 2 domain is essential for retrovirus restriction by TRIM5α, but its specific function is unknown. We show here that the B-box 2 domain mediates higher-order self-association of TRIM5α_rh oligomers. This self-association increases the efficiency of TRIM5α binding to the retroviral capsid, thus potentiating restriction of retroviral infection. The contribution of the B-box 2 domain to cooperative TRIM5α association with the retroviral capsid explains the conditional nature of the restriction phenotype exhibited by some B-box 2 TRIM5α mutants; the potentiation of capsid binding that results from B-box 2-mediated self-association is essential for restriction when B30.2(SPRY) domain-mediated interactions with the retroviral capsid are weak. Thus, B-box 2-dependent higher-order self-association and B30.2(SPRY)-dependent capsid binding represent complementary mechanisms whereby sufficiently dense arrays of capsid-bound TRIM5α proteins can be achieved.