Comparative analysis of plant genomes allows the definition of the "Phytolongins": a novel non-SNARE longin domain protein family

Our comparative genome survey identified a novel family of longin-related proteins, dubbed the "Phytolongins" because they are specific to land plants. Phytolongins share with longins the N-terminal longin domain and the C-terminal transmembrane domain; however, in the central region, the SNARE motif is replaced by a novel region. Phylogenetic analysis pinpoints the Phytolongins as a derivative of the plant specific VAMP72 longin sub-family and allows elucidation of Phytolongin evolution."Longins" have been defined as R-SNAREs composed of both a longin domain and a SNARE motif. However, expressed gene isoforms and splice variants of longins are examples of non-SNARE motif containing longins. The discovery of Phytolongins, a family of non-SNARE longin domain proteins, together with recent evidence on the conservation of the longin-like fold in proteins involved in both vesicle fusion (e.g. the Trs20 tether) and vesicle formation (e.g. σ and μ adaptin) highlight the importance of the longin-like domain in protein trafficking and suggest that it was one of the primordial building blocks of the eukaryotic membrane-trafficking machinery.Membrane-trafficking is a crucial process in eukaryotic cells. In recent years, the combination of structural biology, molecular cell biology and bio-informatics has allowed the definition of many of the key proteins families involved. Genome-wide analyses of both animals and plants, known to possess complex and tightly regulated protein-trafficking systems, have shown extensive sets of such membrane-trafficking protein machinery [1,2]. Among these, the soluble NSF attachment protein receptors (SNAREs) play a central role in the control of membrane fusion and of protein and lipid traffic [3,4]. SNAREs have been divided into major groups based on either their presence in the vesicle (v-SNAREs) or target membrane (t-SNAREs) or based on the presence of a conserved critical residue in the 0 polar layer, either arginine (R-SNAREs) or glutamine