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Publication Abstract from PubMed

SPH proteins are a large family of small, disulphide-bonded, secreted proteins, initially found in the self-incompatibility response in the field poppy (Papaver rhoeas) , but now known to be widely distributed in plants, many containing multiple members of this protein family. Using the Origami strain of E. coli , we expressed one member of this family, SPH15 from Arabidopsis thaliana , as a folded thioredoxin-fusion protein and purified it from the cytosol. The fusion protein was cleaved and characterised by analytical ultracentrifugation, circular dichroism, and NMR spectroscopy. This showed that SPH15 is monomeric and temperature stable, with a beta-sandwich structure. The four strands in each sheet have the same topology as the unrelated proteins; human transthyretin, bacterial TSSJ, and pneumolysin, with no discernable sequence similarity. The NMR-derived structure was compared with a de novo model, made using a new deep learning algorithm based on co-evolution/correlated mutations, DeepCDPred, validating the method. The DeepCDPred de novo method and homology modelling to SPH15 were then both used to derive models of the 3D structure of the three known PrsS proteins from P. rhoeas , which have only 15-18% sequence homology to SPH15. The DeepCDPred method gave models with lower Discreet Optimised Protein Energy (DOPE) scores than the homology models. Three loops at one end of the poppy structures are postulated to interact with their respective pollen receptors to instigate programmed cell death in pollen tubes.

Structure of SPH (Self-Incompatibility Protein Homologue) proteins, a widespread family of small, highly stable, secreted proteins.,Rajasekar K, Ji S, Coulthard RJ, Ride JP, Reynolds GL, Winn PJ, Wheeler MP, Hyde EI, Smith LJ Biochem J. 2019 Feb 19. pii: BCJ20180828. doi: 10.1042/BCJ20180828. PMID:30782970^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.