Proteins - 2019 - Wu - Cardosin A endocytosis mediated by integrin leads to lysosome leakage and apoptosis of epithelial.pdf

Cardosin A is an aspartic protease present in large amount in the pistils of cardoon flowers. This
protease is known to contain an -Arg-Gly-Asp- (RGD) motif located on the molecular surface. In
this study, we found that isolated recombinant cardosin A attached to human epithelial cells
A549, mediated by the binding of its RGD motif to cell surface integrins. The cell bound cardosin
A was internalized to endosomes and lysosomes and triggered the permeability of lysosomal
membrane leading to apoptosis of the epithelial cells. These events are identical to those
observed for three RGD-containing aspartic proteases, Saps 4-6, secreted by Candida albicans.
Such a process, which has been called the Trojan Horse mechanism, is believed to benefit the
invasion of C. albican into the epithelium of the host. The location of the RGD motifs of cardosin
A and Saps 4-6 are on the opposite ends of the homologous three-dimensional structures,
suggesting that the Trojan Horse mechanism is insensitive to the RGD position. Current finding
also suggests that cardosin A may have a defensive function against the ingestion of cardoon
flowers by human, insects, and other herbivores.

菜蓟蛋白酶A（Cardosin A）是一种天冬氨酸蛋白酶（aspartic protease），大量存在于菜蓟（cardoon）花的雌蕊中。已知该蛋白酶的分子表面带有一段精氨酸-甘氨酸-天冬氨酸（Arg-Gly-Asp, RGD）基序。本研究发现，重组分离得到的菜蓟蛋白酶A可通过其RGD基序与细胞表面整合素（integrins）结合，从而附着于人肺泡上皮细胞A549。结合于细胞表面的菜蓟蛋白酶A会被内吞进入内体（endosomes）与溶酶体（lysosomes），并触发溶酶体膜通透性改变，进而引发上皮细胞凋亡。上述过程与白色念珠菌（Candida albicans）分泌的三种含RGD基序的天冬氨酸蛋白酶Saps 4-6所介导的效应完全一致。这一被称为“特洛伊木马（Trojan Horse）”机制的过程，被认为可促进白色念珠菌侵入宿主上皮组织。菜蓟蛋白酶A与Saps 4-6的RGD基序在二者同源三维结构中位于相反的两端，这提示特洛伊木马机制不受RGD基序位置的影响。本研究的最新发现还表明，菜蓟蛋白酶A可能具备抵御人类、昆虫及其他草食动物取食菜蓟花的防御功能。