TCR-induced sumoylation of the kinase PKC-θ controls T cell synapse organization and T cell activation.

Sumoylation regulates many cellular processes, but its role in signaling via the T cell antigen receptor (TCR) remains unknown. We found that the kinase PKC-θ was sumoylated upon costimulation with antigen or via the TCR plus the coreceptor CD28, with Lys325 and Lys506 being the main sumoylation sites. We identified the SUMO E3 ligase PIASxß as a ligase for PKC-θ. Analysis of primary mouse and human T cells revealed that sumoylation of PKC-θ was essential for T cell activation. Desumoylation did not affect the catalytic activity of PKC-θ but inhibited the association of CD28 with PKC-θ and filamin A and impaired the assembly of a mature immunological synapse and central co-accumulation of PKC-θ and CD28. Our findings demonstrate that sumoylation controls TCR-proximal signaling and that sumoylation of PKC-θ is essential for the formation of a mature immunological synapse and T cell activation.

Characterisation of amphioxus protein kinase C-δ/θ reveals a unique proto-V3 domain suggesting an evolutionary mechanism for PKC-θ unique V3.

A primitive adaptive immune system has recently been suggested to be present in a basal chordate amphioxus (Branchiostoma belcheri, Bb), making it an ideal model for studying the origin of adaptive immune. The novel protein kinase C isoform PKC-θ, but not its closest isoform PKC-δ, plays a critical role for mammalian T-cell activation via translocation to immunological synapse (IS) mediated by a unique PKC-θ V3 domain containing one PxxP motif. To understand the evolution of this unique PKC-θ V3 domain and the primitive adaptive immune system in amphioxus, we comparatively studied the orthologs of PKC-δ and -θ from amphioxus and other species. Phylogenetic analysis showed BbPKC-δ/θ to be the common ancestor of vertebrate PKC-δ and PKC-θ, with a V3 domain containing two PxxP motifs. One motif is conserved in both zebrafish and mammalian PKC-θ but is absent in PKC-δ V3 domain of these species, and has already emerged in drosophila PKC-δ. The other non-conserved motif emerged in BbPKC-δ/θ, and only retained in Danio rerio PKC-δ (DrPKC-δ) but lost in mammalian PKC-δ and -θ. Comparative analyses of the sequence and function of BbPKC-δ/θ, DrPKC-δ, DrPKC-θ and Homo sapiens PKC-θ (HsPKC-θ) in IS translocation and T-cell receptor (TCR)-induced NF-κB activation revealed that retention of the conserved PxxP motif and loss of the non-conserved PxxP motif in mammalian PKC-θ and loss of both PxxP motifs in mammalian PKC-δ accomplish the unique function of PKC-θ in T cells. Together, this study suggests an evolutionary mechanism for PKC-θ unique V3 and reveals BbPKC-δ/θ is the common ancestor of PKC-δ and -θ with a functional proto-V3 domain, supplying new evidence for the existence of primitive adaptive immune system in amphioxus.

TCR-Induced Tyrosine Phosphorylation at Tyr270 of SUMO Protease SENP1 by Lck Modulates SENP1 Enzyme Activity and Specificity.

Small ubiquitin-like modifier (SUMO) modification plays an important regulatory role in T cell receptor (TCR) signaling transduction. SUMO-specific proteases (SENPs) have dual-enzyme activities; they can both process SUMO precursors as endopeptidases and participate in SUMO deconjugation as isopeptidases. It remains unclear how the SUMO system, especially SENP1, is regulated by TCR signaling. Here, we show that Lck phosphorylates tyrosine 270 (Y270) of SENP1 upon TCR stimulation, indicating that SENP1 is a substrate of Lck. In vitro endopeptidase activity analysis showed that mutating SENP1 Y270 to either phenylalanine (F) to mimic the phosphorylation-defective state or to glutamate (E) to mimic the negative charge of tyrosine phosphorylation in the enzyme microenvironment did not change its endopeptidase activity towards pre-SUMO1. However, SENP1 Y270E but not Y270F mutation exhibited decreased endopeptidase activity towards pre-SUMO3. Through in vivo isopeptidase activity analysis by rescue expression of SENP1 and its Y270 mutants in a SENP1 CRISPR knockout T cell line, we found that SENP1 Y270F downregulated its isopeptidase activity towards both SUMO1 and SUMO2/3 conjugation by reducing SENP1 binding with sumoylated targets. While overexpression of SENP1 inhibited TCR-induced IL-2 production, overexpression of SENP1 Y270F enhanced it instead. In summary, TCR-induced Y270 phosphorylation of SENP1 may promote its isopeptidase activity and specifically decrease its endopeptidase activity against pre-SUMO3, which finely tunes activation of T cells.

T Cell Receptor (TCR)-Induced PLC-γ1 Sumoylation via PIASxß and PIAS3 SUMO E3 Ligases Regulates the Microcluster Assembly and Physiological Function of PLC-γ1.

The SUMO modification system plays an important role in T cell activation, yet how sumoylation regulates TCR-proximal signaling remains largely unknown. We show here that Phospholipase C-γ1 (PLC-γ1) is conjugated by SUMO1 at K54 and K987 upon TCR stimulation and that K54 sumoylation is pivotal for PLC-γ1-mediated T cell activation. We further demonstrate that TCR-induced K54 sumoylation of PLC-γ1 significantly promotes the formation of PLC-γ1 microclusters and the association of PLC-γ1 with the adaptor proteins SLP76 and Gads, but only slightly affects the phosphorylation of PLC-γ1 on Y783, which determines the enzyme catalytic activity. Moreover, upon TCR stimulation, the SUMO E3 ligases PIASxß and PIAS3 both interact with PLC-γ1 and cooperate to sumoylate PLC-γ1, facilitating the assembly of PLC-γ1 microclusters. Together, our findings reveal a critical role of PLC-γ1 K54 sumoylation in PLC-γ1 microcluster assembly that controls PLC-γ1-mediated T cell activation, suggesting that sumoylation may have an important role in the microcluster assembly of TCR-proximal signaling proteins.