Spooner Robert A, Watson Peter D, Marsden Catherine J, Smith Daniel C, Moore Katherine A H, Cook Jonathon P, Lord J Michael, Roberts Lynne M
Molecular Cell Biology, Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, UK.
Biochem J. 2004 Oct 15;383(Pt 2):285-93. doi: 10.1042/BJ20040742.
Cells expressing ricin B chain within the secretory pathway are significantly more resistant to intoxication by ricin holotoxin but not to other cytotoxins that exploit similar endocytic routes to the cytosol. Furthermore, cells expressing the related B chain of abrin are protected against both incoming abrin and ricin. These phenotypes can be correlated with the abilities of the respective B chains to form disulphide-linked A-B holotoxins, since abrin B chain forms heterodimers with either abrin or ricin A chains, whereas ricin B chain forms heterodimers with ricin A chain only. In the ricin B-expressing cells, this newly made lectin disappears with biphasic kinetics comprising a retention phase followed by slow turnover and disposal after disengagement from calnexin cycle components. Interference with ricin cytotoxicity occurs during the early retention phase when ricin B chain is associated with PDI (protein disulphide-isomerase). The data show that retrotranslocation of incoming toxin is impeded by PDI-catalysed formation of heterodimers between endogenous B and A chains derived from reduced holotoxin, thus proving that reduction of ricin occurs in the endoplasmic reticulum. In contrast with other toxins, ricin does not appear to require either proteolytic cleavage or unfolding for PDI-catalysed reduction.
在分泌途径中表达蓖麻毒素B链的细胞对蓖麻毒素全毒素的中毒具有显著更高的抗性，但对利用类似胞吞途径进入胞质溶胶的其他细胞毒素则没有抗性。此外，表达相思子毒素相关B链的细胞对进入的相思子毒素和蓖麻毒素均有保护作用。这些表型与各自B链形成二硫键连接的A-B全毒素的能力相关，因为相思子毒素B链与相思子毒素或蓖麻毒素A链形成异二聚体，而蓖麻毒素B链仅与蓖麻毒素A链形成异二聚体。在表达蓖麻毒素B链的细胞中，这种新合成的凝集素以双相动力学消失，包括一个保留阶段，随后是缓慢的周转和从钙连蛋白循环成分脱离后的处理。当蓖麻毒素B链与蛋白质二硫键异构酶（PDI）结合时，在早期保留阶段会干扰蓖麻毒素的细胞毒性。数据表明，进入的毒素的逆向转运受到PDI催化的内源性B链和来自还原全毒素的A链之间异二聚体形成的阻碍，从而证明蓖麻毒素在内质网中发生还原。与其他毒素不同，蓖麻毒素似乎不需要蛋白水解切割或展开就能进行PDI催化的还原。
