BACKGROUND Capsid (C) protein plays an important role in the replication of classical swine fever virus (CSFV). The ubiquitin proteasome system (UPS) involves in replication of many viruses via modulation of viral proteins. The relationship of CSFV with UPS is poorly understood and the impact of 26S proteasome on C protein has never been reported before. METHODS In this study, fused C protein with an EGFP tag is expressed in PK-15 and 3D4/2 cells. MG132 and 3-methyladenine (3-MA) are used to detect the roles of 26S proteasome and autophagolysosome in expression levels of C protein. Truncated and mutant C proteins are used to find the exact residues responsible for the degradation of C protein. Immunoprecipitaion is performed to find whether C protein is ubiquitinated or not. RESULTS C-EGFP protein expresses in a cleaved form at a low level and is degraded by 26S proteasome which could be partly inhibited by MG132. C-terminal residues play more important roles in the degradation of C protein than N-terminal residues. Residues 260 to 267, especially M260 and L261, are crucial for the degradation. In addition, C-terminal residues 262 to 267 determine cleavage efficiency of C protein. CONCLUSIONS CSFV C protein is degraded by 26S proteasome in a ubiquitin-independent manner. Last 8 residues at C-terminus of immature C protein play a major role in proteasomal degradation of CSFV C protein and determine the cleavage efficiency of C protein by signal peptide peptidase (SPP). Our findings provide valuable help for fully understanding degradation process of C protein and contribute to fully understanding the role of C protein in CSFV replication.

中文翻译：

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C端残基在经典猪瘟病毒衣壳蛋白降解中的重要作用。

背景技术衣壳（C）蛋白在经典猪瘟病毒（CSFV）的复制中起重要作用。泛素蛋白酶体系统（UPS）涉及通过调节病毒蛋白来复制许多病毒。人们对CSFV与UPS的关系了解甚少，而且以前从未报道过26S蛋白酶体对C蛋白的影响。方法在这项研究中，带有EGFP标签的融合C蛋白在PK-15和3D4 / 2细胞中表达。MG132和3-甲基腺嘌呤（3-MA）用于检测26S蛋白酶体和自噬体在C蛋白表达水平中的作用。使用截短的和突变的C蛋白来查找导致C蛋白降解的确切残基。进行免疫沉淀以发现C蛋白是否被泛素化。结果C-EGFP蛋白以低水平的切割形式表达，并被26S蛋白酶体降解，而MG132可能部分抑制了它的降解。C末端残基在C蛋白的降解中比N末端残基更重要。残留物260至267，尤其是M260和L261，对于降解至关重要。另外，C末端残基262至267确定C蛋白的切割效率。结论26S蛋白酶体以不依赖泛素的方式降解了CSFV C蛋白。未成熟的C蛋白的C末端的最后8个残基在CSFV C蛋白的蛋白酶体降解中起主要作用，并确定C蛋白被信号肽肽酶（SPP）的切割效率。