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 but not its closest isoform plays a critical role for mammalian T-cell activation via translocation to immunological synapse (IS) mediated by a unique V3 domain containing one PxxP motif. To understand the evolution of this unique duanyu1531-θ V3 domain and the primitive adaptive immune system in amphioxus, we comparatively studied the orthologs of and -θ from amphioxus and other species. Phylogenetic analysis showed to be the common ancestor of vertebrate duanyu1531-δ and duanyu1531-θ, with a V3 domain containing two PxxP motifs. One motif is conserved in both zebrafish and mammalian duanyu1531-θ but is absent in duanyu1531-δ V3 domain of these species, and has already emerged in drosophila The other non-conserved motif emerged in and only retained in Danio rerio duanyu1531-δ but lost in mammalian duanyu1531-δ and -θ. Comparative analyses of the sequence and function of Bbduanyu1531-δ/θ, and Homo sapiens duanyu1531-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 duanyu1531-θ and loss of both PxxP motifs in mammalian duanyu1531-δ accomplish the unique function of duanyu1531-θ in T cells. Together, this study suggests an evolutionary mechanism for duanyu1531-θ unique V3 and reveals Bbduanyu1531-δ/θ is the common ancestor of duanyu1531-δ and -θ with a functional proto-V3 domain, supplying new evidence for the existence of primitive adaptive immune system in amphioxus.

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