Important roles of C-terminal residues in degradation of capsid protein of classical swine fever virus
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RESEARCH
Open Access
Important roles of C-terminal residues in degradation of capsid protein of classical swine fever virus Yuming Chen†, Erpeng Zhu†, Shuangqi Fan, Hongxing Ding, Shengming Ma, Mengjiao Zhu, Shaofeng Deng, Jinding Chen* and Mingqiu Zhao*
Abstract 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 3methyladenine (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. Keywords: Classical swine fever virus (CSFV), C protein, 26S proteasome, Degradation, Cleavage
Background The ubiquitin proteasome system (UPS) is consist of the ubiquitin cascade reaction system and 26S proteasome. The UPS and lysosome-dependent autophagy system are two major ways of protein degradation in cells and they play significant roles in various cell activities by modulation of key protein levels [1–5]. Ubiquitination is a kind of posttranslational protein modification in which ubiquitin is added to the substrate protein by covalent bonding [6]. Most proteins degraded by the 26S proteasome are * Correspondence: [email protected]; [email protected] † Yuming Chen and Erpeng Zhu contributed equally to this work. College of Veterinary Medicine, South China Agricultural University, No. 483, Wushan Road; Tianhe District, Guangzhou 510642, People’s Republic of China
modified by ubiquitination, while some are degraded in a ubiquitin-independent manner with the involvement of proteasome activator 28 (PA28) or other proteins [7, 8]. Ubiquitination mostly occurs on lysine residues and sometimes on cysteine, seri
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