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Staphylococcus aureus secretes the SSL7 protein as part of its immune evasion strategy. The protein binds both complement C5 and IgA, yet it is unclear whether SSL7 cross-links these two proteins and, if so, what purpose this serves the pathogen. We have isolated a stable IgA-SSL7-C5 complex, and our crystal structure of the C5-SSL7 complex confirms that binding to C5 occurs exclusively through the C-terminal β-grasp domain of SSL7 leaving the OB domain free to interact with IgA. SSL7 interacts with C5 >70 Å from the C5a cleavage site without inducing significant conformational changes in C5, and efficient inhibition of convertase cleavage of C5 is shown to be IgA dependent. Inhibition of C5a production and bacteriolysis are all shown to require C5 and IgA binding while inhibition of hemolysis is achieved by the C5 binding SSL7 β-grasp domain alone. These results provide a conceptual and structural basis for the development of a highly specific complement inhibitor preventing only the formation of the lytic membrane attack complex without affecting the important signaling functions of C5a. Complement activation triggers cleavage of the three homologous 185-to 200-kDa proteins C3, C4, and C5. Three activation pathways converge on C3 cleavage to C3b (6). Activation by the alternative pathway (AP) results from spontaneous hydrolysis of an internal thioester bond in C3 or by deposition of properdin on an appropriate surface (7), whereas the classical pathway (CP) and lectin-mediated pathways are activated by surface-bound immune complexes or mannan binding lectins/ficolins, respectively. Activation of the CP or the lectin pathway generates the surface-bound C3 convertase (a proteolytic enzyme cleaving C3) C4b2a, whereas the AP generates the C3 convertase C3bBb. Both may recruit an additional C3b molecule to form the CP C5 convertase C4b2a3b or the AP C5 convertase C3bBb3b (8, 9), which cleaves C5 to generate the large fragment C5b and the anaphylatoxin C5a. This binds with high affinity to the C5a receptor (CD88) on myeloid cells triggering G protein (Gα I and Gα 16 )-mediated cell activation, chemotaxis, respiratory burst, and release of proinflammatory mediators (10). C5b quickly associates with C6, C7, C8, and multiple molecules of C9 to form the membrane attack complex (MAC) that results in rapid cell lysis (11). Elevated levels of C5a are implicated in a wide range of inflammatory disorders, such as rheumatoid arthritis, ischemia/reperfusion injury, sepsis, and fibrotic conditions (12). MAC deposition through C5b on erythrocytes results in destruction of these cells in the hemolytic disease paroxysmal nocturnal hemoglobinuria (PNH) (13). In addition, excessive MAC formation is linked with the pathophysiology of conditions such as antibody-mediated transplant rejection (14), inflammatory neuropathies (15) and multiple sclerosis (16).Given its importance to innate immune clearance, pathogens have developed many strategies to prevent complement activation (17). The Staphylococcal Superantigen-Like protein 7 (SSL7)...

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Recent advances in open-water measurements suggest significant temporal and spatial variability of gross primary production (GPP), net ecosystem production (NEP), and respiration (R) with implications for understanding carbon cycling in lakes. This study applied high-frequency depth profiles in three stratified lakes of different trophic status to investigate (1) the importance of vertical variations in metabolic rates, (2) the effects of changes in the depth of the mixed layer (Z mix ) and the photic zone (Z eu ), and (3) the photoacclimative responses of the aquatic autotrophs to changes in these conditions. Taking account of vertical differences in metabolism improved the reliability of whole-areal NEP estimates during stratification. Whereas the hypolimnion was always heterotrophic, and the epilimnion was mostly autotrophic, the metalimnion had NEP . 0 when Z eu . Z mix . Although most of GPP and R occurred in the epilimnion, between 0% and 20% of GPP and 4% and 37% of R took place in the metalimnion. Areal metabolic estimates based on surface measurements deviated up to 60% for GPP and 80% for R when Z eu . Z mix . The vertical variability in metabolism was driven by available light in both the epi-and metalimnion. Coupling between GPP and R was low in all layers and indicated increasing background R with depth. Light utilization efficiency was significantly higher under low light conditions, indicating photophysiological acclimation of phytoplankton to decreasing light in the metalimnion.