Laboratory of Malaria and Vector Research, NIH-NIAID, Rockville, Maryland, USA.
Laboratory of Malaria and Vector Research, NIH-NIAID, Rockville, Maryland, USA.
J Biol Chem. 2021 Jan-Jun;296:100083. doi: 10.1074/jbc.RA120.015230. Epub 2020 Nov 24.
Inhibition of the alternative pathway (AP) of complement by saliva from Anopheles mosquitoes facilitates feeding by blocking production of the anaphylatoxins C3a and C5a, which activate mast cells leading to plasma extravasation, pain, and itching. We have previously shown that albicin, a member of the SG7 protein family from An. Albimanus, blocks the AP by binding to and inhibiting the function of the C3 convertase, C3bBb. Here we show that SG7.AF, the albicin homolog from An. freeborni, has a similar potency to albicin but is more active in the presence of properdin, a plasma protein that acts to stabilize C3bBb. Conversely, albicin is highly active in the absence or presence of properdin. Albicin and SG7.AF stabilize the C3bBb complex in a form that accumulates on surface plasmon resonance (SPR) surfaces coated with properdin, but SG7.AF binds with lower affinity than albicin. Albicin induces oligomerization of the complex in solution, suggesting that it is oligomerization that leads to stabilization on SPR surfaces. Anophensin, the albicin ortholog from An. stephensi, is only weakly active as an inhibitor of the AP, suggesting that the SG7 family may play a different functional role in this species and other species of the subgenus Cellia, containing the major malaria vectors in Africa and Asia. Crystal structures of albicin and SG7.AF reveal a novel four-helix bundle arrangement that is stabilized by an N-terminal hydrogen bonding network. These structures provide insight into the SG7 family and related mosquito salivary proteins including the platelet-inhibitory 30 kDa family.
按蚊唾液中的补体替代途径（AP）抑制物通过阻断过敏毒素 C3a 和 C5a 的产生来促进蚊子吸血，这些过敏毒素会激活肥大细胞导致血浆外渗、疼痛和瘙痒。我们之前已经表明，来自 An. Albimanus 的 SG7 蛋白家族成员 albicin 通过与 C3 转化酶 C3bBb 结合并抑制其功能来阻断 AP。在这里，我们表明来自 An. freeborni 的 albicin 同源物 SG7.AF 具有与 albicin 相似的效力，但在存在调理素（一种稳定 C3bBb 的血浆蛋白）的情况下更具活性。相反，albicin 在没有或存在调理素的情况下高度活跃。Albicin 和 SG7.AF 以一种在表面等离子体共振（SPR）表面上积累的形式稳定 C3bBb 复合物，该复合物涂有调理素，但 SG7.AF 的结合亲和力低于 albicin。Albicin 在溶液中诱导复合物的寡聚化，表明寡聚化导致在 SPR 表面的稳定。来自 An. stephensi 的 albicin 直系同源物 Anophensin 作为 AP 抑制剂的活性较弱，这表明 SG7 家族在该物种和包含非洲和亚洲主要疟疾媒介的亚属 Cellia 中的其他物种中可能发挥不同的功能作用。Albicin 和 SG7.AF 的晶体结构揭示了一种新颖的四螺旋束排列，由 N 端氢键网络稳定。这些结构为 SG7 家族和相关的蚊子唾液蛋白（包括血小板抑制 30 kDa 家族）提供了深入了解。
