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-theta, but not its closest isoform PKC-delta, plays a critical role for mammalian T-cell activation via translocation to immunological synapse (IS) mediated by a unique PKC-8 V3 domain containing one PxxP motif. To understand the evolution of this unique PKC-theta V3 domain and the primitive adaptive immune system in amphioxus, we comparatively studied the orthologs of PKC-delta and -theta from amphioxus and other species. Phylogenetic analysis showed BbPKC-6/0 to be the common ancestor of vertebrate PKC-S and PKC-8, with a V3 domain containing two PxxP motifs. One motif is conserved in both zebrafish and mammalian PKC-0 but is absent in PKC-delta V3 domain of these species, and has already emerged in drosophila PKC-delta. The other non-conserved motif emerged in BbPKC-delta/theta, and only retained in Danio rerio PKC-delta (DrPICC-delta) but lost in mammalian PKC-delta and -theta. Comparative analyses of the sequence and function of BbPKC-delta/theta, DrPKC-delta, DrPKC-theta and Homo sapiens PKC-theta (HsPKC-theta) in IS translocation and T-cell receptor (TCR)-induced NF-kappa Beta activation revealed that retention of the conserved PxxP motif and loss of the non-conserved PxxP motif in mammalian PKC-theta and loss of both PxxP motifs in mammalian PKC-delta accomplish the unique function of PKC-theta in T cells. Together, this study suggests an evolutionary mechanism for PKC-theta unique V3 and reveals BbPKC-delta/theta is the common ancestor of PKC-delta and -theta with a functional proto-V3 domain, supplying new evidence for the existence of primitive adaptive immune system in amphioxus.