研究牛痘病毒鞘膜蛋白在病毒進入細胞過程
中所扮演的角色
牛痘病毒的宿主範圍相當廣泛，幾乎能夠感染所有的細胞，故推測牛
痘病毒進行感染時，所利用的細胞受體（ cellular receptor ）分布應該
會十分普遍。在本篇論文中，我們根據下述的實驗結果，證實牛痘病
毒會經由細胞表面的 heparan sulfate 以感染細胞。（一） soluble hepari
n 能夠排擠（ compete ）宿主細胞和病毒之間的結合作用；（二）細胞
經過 chlorate 處理後，會抑制細胞表面 heparan sulfate 進行 sulfation ，
而造成此類細胞不易被病毒感染；（三）純化的牛痘病毒能夠直接結
合至 heparin beads 。除此之外，我們發現 IMV 之鞘膜蛋白質 A27L 、
D8L 蛋白質均會附著至細胞表面 heparan sulfate ，並且證明這兩種蛋白
質為牛痘病毒入侵細胞的附著蛋白質（ attachment proteins ）。其中 D
8L 蛋白質為主要和 heparan sulfate 結合的鞘膜蛋白質，牛痘病毒會藉
著 D8L 蛋白質而附著至細胞表面； A27L 蛋白質則為較次要的 heparan
sulfate binding protein ，只有在 D8L 蛋白質不存在的情況下，才會影響
病毒附著至細胞表面。因此， A27L 蛋白質在病毒入侵細胞過程中
，所扮演的角色在 post-binding stage 比較重要，研究結果顯示其 N 端
的 a.a.21-32 的部分負責與細胞表面的 heparan sulfate 作用，藉此方能
催化細胞融合（ cell fusion ）的產生。
Investigation of Vaccinia Virus Attachment P
roteins Involved in Virus Entry
Vaccinia virus has a wide host range and infects mammalian cells of many diffe rent species. This suggests that the cell surface receptors for vaccinia virus are ubiquitously expressed and highly conserved. Alternatively, different rec eptors are used for vaccinia virus infection of different cell types. We previ ously shown that vaccinia virus infection to BSC40 cells was blocked by solubl e heparin, suggesting that cell surface heparan sulfate mediates vaccinia viru s binding. BSC40 cells treated with sodium chlorate to prevent sulfation of ce ll surface GAGs became more resistant to vaccinia virus infection indicating t he negative charges contributed by sulfate groups are important for virus infe ction. We investigated the structure of A27L protein that is required for its binding to heparan sulfate on cells. A mutant A27L protein, named D-A27L,which removed a cluster of 12 a.a.s rich in basic residues was constructed. In cont rast to soluble A27L protein, D-A27L protein did not bind to heparin in vitro.
D-A27L protein also did not compete with soluble A27L protein for binding to
heparan sulfate on cells. Most importantly, soluble A27L but not D-A27L prote
in interfered with vaccinia virus infection. Virus binding assays revealed tha
t the interference of A27L protein was at a postbinding step. Taken together,
these data demonstrated the N-terminal region of A27L protein functions as a G
AGs-binding domain and is critical for its function. Since A27L protein w
as thought to be involved in cell fusion under acid treatment, we investigated
if cell surface GAGs also participate in A27L-dependent fusion. Mouse L cells
infected by vaccinia virus developed cell fusion after brief exposure to acid
ic environment. Soluble A27L protein blocked cell fusion whereas D-A27L protei
n did not suggesting the GAGs binding domain plays a role in A27L-mediated cel
l fusion. We also performed the converse experiment in which sog9 cells, a GAG
s-free mutant cells derived from L cells, were infected with vaccinia virus an
d no fusion was observed. The results demonstrated that A27L-mediated cell fus
ion is triggered by its interaction with cell surface GAGs. In summary, our d
ata indicate that during vaccinia virus infection, A27L protein could interact
with cell surface haparan sulfate through the N-terminal region. This interac
tion directly or indirectly triggers membrane fusion, resulting in virus entry
. In addition, vaccinia virus may have gene redundancy in that additional vira
l proteins such as D8L also binds to haparan sulfate as well.
