Kozak S L, Hoatlin M E, Ferro F E, Majumdar M K, Geib R W, Fox M T, Kabat D
Department of Biochemistry and Molecular Biology, School of Medicine, Oregon Health Sciences University, Portland 97201-3098.
J Virol. 1993 May;67(5):2611-20. doi: 10.1128/JVI.67.5.2611-2620.1993.
The env gene of Friend spleen focus-forming virus (SFFV) encodes a membrane glycoprotein (gp55) that is inefficiently (3 to 5%) processed from the rough endoplasmic reticulum to form a larger dimeric plasma membrane derivative (gp55p). Moreover, the SFFV env glycoprotein associates with erythropoietin receptors (EpoR) to cause proliferation of infected erythroblasts [J.-P. Li, A. D. D'Andrea, H. F. Lodish, and D. Baltimore, Nature (London) 343:762-764, 1990]. Interestingly, the mitogenic effect of SFFV is blocked in mice homozygous for the Fv-2r resistance gene, but mutant SFFVs can overcome this resistance. Recent evidence suggested that these mutants contain partial env deletions that truncate the membrane-proximal extracellular domain of the encoded glycoproteins (M. H. Majumdar, C.-L. Cho, M. T. Fox, K. L. Eckner, S. Kozak, D. Kabat, and R. W. Geib, J. Virol. 66:3652-3660, 1992). Mutant BB6, which encodes a gp42 glycoprotein that has a large deletion in this domain, causes erythroblastosis in DBA/2 (Fv-2s) as well as in congenic D2.R (Fv-2r) mice. Analogous to gp55, gp42 is processed inefficiently as a disulfide-bonded dimer to form cell surface gp42p. Retroviral vectors with SFFV and BB6 env genes have no effect on interleukin 3-dependent BaF3 hematopoietic cells, but they cause growth factor independency of BaF3/EpoR cells, a derivative that contains recombinant EpoR. After binding 125I-Epo to surface EpoR on these factor-independent cells and adding the covalent cross-linking reagent disuccinimidyl suberate, complexes that had immunological properties and sizes demonstrating that they consisted of 125I-Epo-gp55p and 125I-Epo-gp42p were isolated from cell lysates. Contrary to a previous report, SFFV or BB6 env glycoproteins did not promiscuously activate other members of the EpoR superfamily. Although the related env glycoproteins encoded by dualtropic murine leukemia viruses formed detectable complexes with EpoR, strong mitogenic signalling did not ensue. Our results indicate that the SFFV and BB6 env glycoproteins specifically activate EpoR; they help to define the glycoprotein properties important for its functions; and they strongly suggest that the Fv-2 leukemia control gene encodes an EpoR-associated regulatory factor.
弗瑞德脾脏灶形成病毒（SFFV）的env基因编码一种膜糖蛋白（gp55），该蛋白从粗面内质网加工形成更大的二聚体细胞膜衍生物（gp55p）的效率较低（3%至5%）。此外，SFFV env糖蛋白与促红细胞生成素受体（EpoR）结合，导致受感染的成红细胞增殖[J.-P. 李、A. D. 安德烈亚、H. F. 洛迪什和D. 巴尔的摩，《自然》（伦敦）343:762 - 764，1990]。有趣的是，SFFV的促有丝分裂作用在Fv - 2r抗性基因纯合的小鼠中被阻断，但突变的SFFV可以克服这种抗性。最近的证据表明，这些突变体包含部分env缺失，这些缺失截断了所编码糖蛋白的膜近端细胞外结构域（M. H. 马俊达、C.-L. 赵、M. T. 福克斯、K. L. 埃克纳、S. 科扎克、D. 卡巴特和R. W. 盖布，《病毒学杂志》66:3652 - 3660，1992）。编码在该结构域有大片段缺失的gp42糖蛋白的突变体BB6，在DBA/2（Fv - 2s）以及同基因的D2.R（Fv - 2r）小鼠中引起成红细胞增多症。与gp55类似，gp42作为二硫键连接的二聚体加工效率低下，形成细胞表面的gp42p。携带SFFV和BB6 env基因的逆转录病毒载体对白细胞介素3依赖的BaF3造血细胞没有影响，但它们导致BaF3/EpoR细胞（一种含有重组EpoR的衍生物）不依赖生长因子。在这些不依赖因子的细胞表面EpoR上结合125I - Epo并添加共价交联剂辛二酸二琥珀酰亚胺酯后，从细胞裂解物中分离出具有免疫特性和大小表明它们由125I - Epo - gp55p和125I - Epo - gp42p组成的复合物。与之前的报道相反，SFFV或BB6 env糖蛋白不会随意激活EpoR超家族的其他成员。尽管双嗜性鼠白血病病毒编码的相关env糖蛋白与EpoR形成了可检测的复合物，但并未引发强烈的促有丝分裂信号。我们的结果表明，SFFV和BB6 env糖蛋白特异性激活EpoR；它们有助于确定对其功能重要的糖蛋白特性；并且它们强烈表明Fv - 2白血病控制基因编码一种与EpoR相关的调节因子。
