Many geminiviruses including members of the genus Begomovirus produce a protein known as C4 or AC4. Whereas C4/AC4 typically consists of over 80 amino acid residues, a few are much shorter. The significance of these shorter C4/AC4 proteins in viral infection and why the virus maintains their abbreviated length is not yet understood. The AC4 of the begomovirus tomato leaf curl Hsinchu virus (ToLCHsV) contains only 65 amino acids, but it extends to 96 amino acids when the natural termination codon is replaced with a normal codon. We discovered that both interrupting and extending AC4 were harmful to ToLCHsV. The extended AC4 (EAC4) also showed a lower ability to promote the infection of the heterologous virus potato virus X than the wildtype AC4. When the wildtype AC4 was fused with yellow fluorescent protein (AC4-YFP), it was predominantly found in chloroplasts, whereas EAC4-YFP was mainly localized to the cell periphery. These results suggest that ToLCHsV's AC4 protein is important for viral infection, and the virus may benefit from its abbreviated length because it may lead to chloroplast localization.

中文翻译：

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二分贝戈莫病毒 AC4 蛋白的正常功能需要大小限制。

许多双生病毒，包括 Begomovirus 属的成员，都会产生一种称为 C4 或 AC4 的蛋白质。虽然 C4/AC4 通常由超过 80 个氨基酸残基组成，但也有一些要短得多。这些较短的 C4/AC4 蛋白在病毒感染中的重要性以及病毒为何保持其缩短的长度尚不清楚。Begomovirus番茄卷曲新竹病毒（ToLCHsV）的AC4仅包含65个氨基酸，但当天然终止密码子被替换为正常密码子时，其AC4延伸至96个氨基酸。我们发现中断和延长 AC4 都对 ToLCHsV 有害。与野生型AC4相比，延伸AC4(EAC4)还表现出较低的促进异源病毒马铃薯病毒X感染的能力。当野生型AC4与黄色荧光蛋白（AC4-YFP）融合时，它主要存在于叶绿体中，而EAC4-YFP主要位于细胞外周。这些结果表明ToLCHsV的AC4蛋白对于病毒感染很重要，并且病毒可能受益于其缩短的长度，因为它可能导致叶绿体定位。