Crystal Structure of the SPOC Domain of the <i>Arabidopsis</i> Flowering Regulator FPA

The Arabidopsis protein FPA controls flowering time by regulating the alternative 3′-end processing of the FLOWERING LOCUS (FLC) antisense RNA. FPA belongs to the split ends (SPEN) family of proteins, which contain N-terminal RNA recognition motifs (RRMs) and a SPEN paralog and ortholog C-terminal (SPOC) domain. The SPOC domain is highly conserved among FPA homologs in plants, but the conservation with the domain in other SPEN proteins is much lower. We have determined the crystal structure of Arabidopsis thaliana FPA SPOC domain at 2.7 Å resolution. The overall structure is similar to that of the SPOC domain in human SMRT/HDAC1 Associated Repressor Protein (SHARP), although there are also substantial conformational differences between them. Structural and sequence analyses identify a surface patch that is conserved among plant FPA homologs. Mutations of two residues in this surface patch did not disrupt FPA functions, suggesting that either the SPOC domain is not required for the role of FPA in regulating RNA 3′-end formation or the functions of the FPA SPOC domain cannot be disrupted by the combination of mutations, in contrast to observations with the SHARP SPOC domain.拟南芥FPA蛋白通过调控开花位点C（FLOWERING LOCUS, FLC）反义RNA的可变3'端加工过程，控制植物开花时间。FPA属于剪接末端（split ends, SPEN）蛋白家族，该家族蛋白包含N端RNA识别基序（RNA recognition motifs, RRMs）以及一个SPEN旁系同源与直系同源C端（SPEN paralog and ortholog C-terminal, SPOC）结构域。SPOC结构域在植物FPA同源蛋白中高度保守，但与其他SPEN家族蛋白中的该结构域的保守性显著更低。本研究解析了拟南芥（Arabidopsis thaliana）FPA SPOC结构域的晶体结构，分辨率达2.7埃（Å）。其整体结构与人类SMRT/HDAC1相关阻遏蛋白（SMRT/HDAC1 Associated Repressor Protein, SHARP）的SPOC结构域相似，但二者间也存在显著的构象差异。结构与序列分析鉴定出一个在植物FPA同源蛋白中保守的表面斑块（surface patch）。该表面斑块内两个残基的突变并未破坏FPA的功能，这提示两种可能性：一是FPA在调控RNA 3'端形成的过程中并不需要SPOC结构域，二是此次组合突变无法破坏FPA SPOC结构域的功能，这与针对SHARP SPOC结构域的已有研究观察结果相悖。