乙型肝炎病毒小表面抗原(small hepatitis B virus surface antigen，SHB)在细胞内质网上表达，沿着细胞分泌途径分泌到胞外。为系统分析SHB拓扑结构对SHB表达和分泌的影响，首先通过生物信息学预测临床病毒株HBV C8和8种基因型(A～H)代表株的SHB拓扑结构，发现这些SHB均为四次跨膜蛋白，拥有基本相同的拓扑结构。相对内质网膜而言，SHB的拓扑结构拥有3个内质网腔内区段(Inside1～Inside3)、4个跨膜螺旋区(Tmhelix1～Tmhelix4)和2个内质网膜外区段(Outside1和Outside2)。6种基因型(基因型A、B、C、D、E和G)代表株与病毒株C8的SHB拓扑结构预测结果完全相同，而基因型F和H的SHB有4个区段与C8等不完全一致。通过对C8的SHB拓扑结构各区段进行缺失突变研究，发现Inside1区段不是SHB表达和分泌所必需的；Outside1、Tmhelix2和Inside2区段是SHB表达和分泌所必需的；Tmhelix1和Outside2不是SHB表达所必需的，但为SHB分泌所必需；Tmhelix3和Tmhelix4对SHB表达有重要影响，也是SHB分泌所必需的。进一步对Outside1和Outside2进行小片段(6个氨基酸)的缺失突变研究，发现小片段缺失基本不显著影响SHB的表达，但Outside1的氨基酸55～78及Outside2是SHB分泌所必需的。本研究首次系统性分析了SHB的拓扑结构各区段对SHB表达和分泌的影响，为深入探索SHB结构与功能的关系提供了线索。

The small hepatitis B virus surface antigen (SHB) is expressed on the endoplasmic reticulum (ER) and is secreted along the cellular secretory pathway. To analyze the impact of SHB domains on its expression and secretion, SHB proteins of eight HBV genotypes (A-H) plus a clinical strain C8 were subjected to a domain analysis. All the SHBs have four transmembrane motifs (Tmhelix1-4), three regions in the ER lumen (Inside1-3), and two regions outside of the ER (Outside1-2). The topological regions of SHBs in C8 and genotypes A, B, C, D, E and G were identical, while four topological regions of SHBs in genotypes F and H were slightly different from that in C8. Deletion analysis of SHB topological regions in C8 indicated that Inside1 region was not required for SHB expression and secretion; Outside1, Tmhelix2 and Inside2 regions were required for both SHB expression and secretion; Tmhelix1 and Outside2 regions were dispensable for SHB expression but needed for SHB secretion; Tmhelix3 and Tmhelix4 regions were important for SHB expression and necessary for SHB secretion. Finer deletion (6 amino acids) mapping in Outside1 and Outside2 regions showed that amino acids 55-78 of Outside1 and Outside2 were required for SHB secretion. This is the first systematic analysis of the impact of SHB topological regions on SHB expression and secretion, which has provided valuable clues for future study of the structure-function relationship of SHB.