Structure of SPH (Self-Incompatibility Protein Homologue) Proteins: a Widespread Family of Small, Highly Stable, Secreted Proteins

SPH proteins are a large family of small, disulphide-bonded, secreted proteins, initially found in the self-incompatibility response in the field poppy , but now known to be widely distributed in plants, many containing multiple members of this protein family. Using the Origami strain of , we expressed one member of this family, SPH15 from , 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 model, made using a new deep learning algorithm based on co-evolution/correlated mutations, DeepCDPred, validating the method. The DeepCDPred method and homology modelling to SPH15 were then both used to derive models of the 3D structure of the three known PrsS proteins from , 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. [Abstract copyright: ©2019 The Author(s).

The coordination of heart and gill rhythms in \u3cem\u3eLimulus\u3c/em\u3e

WhenLimulus is exposed to hypoxia both heart rate and ventilation rate decrease together (Fig. 1, Fig. 2A). Hypoxia ultimately leads to cessation of ventilation and concomitant bradycardia. When oxygen is reintroduced into an oxygen free aquarium ventilation resumes rapidly, with a parallel increase in heart rate (Fig. 1, Fig. 2B). Covariation of heart and gill activity similar to that in hypoxia experiments also occurs during the normal respiratory behavior patterns ofLimulus, such as intermittent ventilation, swimming, hyperventilation and gill cleaning. The covariation of heart and ventilation rates is especially evident during transitions of intermittent ventilation (alternating periods of apnea and ventilation, Fig. 3). Covariation is also evident during the large increases in ventilation frequency which occur during hyperventilation and swimming (Fig. 4). Gill cleaning is a centrally determined motor sequence which consists of rhythmic flicking of the inner lobes of a gill plate between the book gill lamellae of the plate on the opposite side. During this behavior there is a marked slowing of the heart rate which is at least as great as the decrease in rate seen during periods of apnea (Figs. 5 and 6). Changes in heart rate associated with ventilatory activity do not appear to be caused by the metabolic demand resulting from such activity (Fig. 7). In addition to frequency covariation of the heart and ventilation rates, there can also be phase coordination of the two rhythms. When the two are close to the same frequency or to harmonic frequencies, the heart often maintains a phase preference with respect to the concurrent gill interval over a considerable period of time (Fig. 8). These results suggest that there are common tonic inputs to both the cardiac ganglion and the central pattern generators for the various ventilatory behaviors, which modulate the frequencies of both simultaneously. Both the frequency covariation and phase communication between the two systems may serve to increase the efficiency of the respiratory-circulatory interactions