Novel nucleocytoplasmic protein O-fucosylation by SPINDLY regulates diverse developmental processes in plants

Abstract

In metazoans, protein O-fucosylation of Ser/Thr residues was only found in secreted or cell surface proteins, and this post-translational modification is catalyzed by ER-localized protein O-fucosyltransferases (POFUTs) in the GT65 family. Recently, a novel nucleocytoplasmic POFUT, SPINDLY (SPY), was identified in the reference plant Arabidopsis thaliana to modify nuclear transcription regulators DELLAs, revealing a new regulatory mechanism for gene expression. The paralog of AtSPY, SECRET AGENT (SEC), is an O-link-N-acetylglucosamine (GlcNAc) transferase (OGT), which O-GlcNAcylates Ser/Thr residues of target proteins. Both AtSPY and AtSEC are tetratricopeptide repeat-domain-containing glycosyltransferases in the GT41 family. The discovery that AtSPY is a POFUT clarified decades of miss-classification of AtSPY as an OGT. SPY and SEC play pleiotropic roles in plant development, and the interactions between SPY and SEC are complex. SPY-like genes are conserved in diverse organisms, except in fungi and metazoans, suggesting that O-fucosylation is a common mechanism in modulating intracellular protein functions.

Figure 1.. Structure comparison among human OGT, Arabidopsis SEC and SPY.

(a) Diagrams of HsOGT, AtSEC and AtSPY. TPRs are in grey. N-terminal catalytic domains, N-Cat, are in cyan. C-terminal catalytic domains, C-Cat, are in blue. (b) 3D structures of HsOGT (PDB ID: 4N3C, containing 4.5-TPRs)[71], and predicted 3D structures of Arabidopsis SEC and SPY using SWISS MODEL[72,73]. The HsOGT crystal structure (PDB ID: 4N3C)[71] was used as scaffold to predict AtSEC and AtSPY structures. The color schemes for HsOGT, AtSEC and AtSPY are as in (a). In (b), UDP-GlcNAc in HsOGT is shown as spheres (in lime-green). In the HsOGT structure in (b), the transitional helix (H3) between TPRs and N-Cat, and the first 2 α-helices (H1 and H2) of N-Cat are highlighted in magenta. The long intervening domain between N-Cat and C-Cat of HsOGT is omitted from the structure because this domain is uniquely present in the animal OGTs. This figure was modified from Zentella et al. [14].

Figure 2.. Model for the opposing roles of O-fucosylation and O-GlcNAcylation of DELLA in regulating plant growth.

(a) O-GlcNAcylation by OGT (SEC). (b) O-Fucosylation by POFUT (SPY). (c) The nuclear growth repressor DELLA proteins are activated by O-Fucosylation, and repressed by O-GlcNAcylation. Each DELLA protein contains an N-terminal DELLA domain and a C-terminal GRAS domain. O-Fucosylation (labeled as F) by SPY may induce the DELLA protein to an open conformation that is a more active growth repressor; this open form promotes binding of the GRAS domain to interacting transcription factors (e.g., BZR1 and PIFs), which leads to down-regulated expression of target genes of BZR1 and PIFs to restrict plant growth. In contrast, O-GlcNAcylation (labeled as G) by SEC may cause the DELLA protein to fold into a closed conformation that is less active because this form reduces its binding affinity to BZR1 and PIFs so that growth-related target genes can be activated. TF, DELLA-interacting transcription factor. The figure (c) was modified from Zentella et al. [14].

Figure 3.. SPY regulates circadian period by inducing PRR5 degradation.

O-Fucosylation (labeled as F) of the transcription repressor PRR5 by SPY promotes PRR5 degradation. The spy mutant has a longer circadian period than WT.