Selective DNA-binding of SP120 (rat ortholog of human hnRNP U) is mediated by arginine-glycine rich domain and modulated by RNA

A human protein heterogeneous ribonucleoprotein U (hnRNP U) also known as Scaffold attachment factor A (SAF-A) and its orthologous rat protein SP120 are abundant and multifunctional nuclear protein that directly binds to both DNA and RNA. The C-terminal region of hnRNP U enriched with arginine and glycine is essential for the interaction with RNA and the N-terminal region of SAF-A termed SAP domain has been ascribed to the DNA binding. We have reported that rat hnRNP U specifically and cooperatively binds to AT-rich DNA called nuclear scaffold/matrix-associated region (S/MAR) although its detailed mechanism remained unclear. In the present study analysis of hnRNP U deletion mutants revealed for the first time that a C-terminal domain enriched with Arg-Gly (defined here as 'RG domain') is predominantly important for the S/MAR-selective DNA binding activities. RG domain alone directly bound to S/MAR and coexistence with the SAP domain exerted a synergistic effect. The binding was inhibited by netropsin, a minor groove binder with preference to AT pairs that are enriched in S/MAR, suggesting that RG domain interacts with minor groove of S/MAR DNA. Interestingly, excess amounts of RNA attenuated the RG domain-dependent S/MAR-binding of hnRNP U. Taken together, hnRNP U may be the key element for the RNA-regulated recognition of S/MAR DNA and thus contributing to the dynamic structural changes of chromatin compartments.