胸腺白血病抗原的亚基结构。

Anundi H, Rask L, Ostberg L, Peterson P A

Biochemistry. 1975 Nov 18;14(23):5046-54. doi: 10.1021/bi00694a003.

EDTA-containing buffer solubilizes thymus leukemia antigens (TLa) from crude thymocyte membrane fractions. The TL antigens consist mainly of molecules of a size similar to immunoglobulin G when gel chromatography analyses were performed under physiological conditions. A single component of TLa was apparent on sucrose density gradient ultracentrifugation of solubilized thymocyte membrane macromolecules as monitored by indirect immunoprecipitation. The sedimentation constant for the TL antigens (5.8 S) was considerably less than that for immunoglobulin G. The gel chromatography and ultracentrifugation data suggest an apparent molecular weight for TLa of about 120000. TLa isolated by indirect immunoprecipitation is composed of two types of polypeptide chains. The smaller subunit was identified as beta2-microglobulin. The larger polypeptide chain carried the alloantigenic determinants and displayed a molecular weight of about 50000 after reduction and alkylation. TLa subjected to molecular weight determination under denaturing conditions was composed of two components. The smaller component was beta2-microglobulin which evidently is linked to the larger polypeptide chain by noncovalent interactions only. The larger component had a size greater than reduced and alkylated immunoglobulin G heavy chains. Upon reduction and alkylation of the latter component its size was reduced and it appeared to have a molecular weight of about 50000. Consequently, TLa is composed of two disulfide linked heavy polypeptide chains and two beta2-microglobulin molecules. TLa solubilized by papain digestion comprises two polypeptide chains, one of which is beta2-microglobulin. The larger 37000-dalton subunit is a fragment of the heavy polypeptide chain. This was demonstrated by digesting solubilized 120000-dalton TLa with papain. The proteolytic fragments obtained were indistinguishable from those directly released from the cell surface by proteolysis. The papain-derived TLa fragment exhibited most if not all the alloantigenic determinants.

含乙二胺四乙酸（EDTA）的缓冲液可溶解粗制胸腺细胞膜组分中的胸腺白血病抗原（TLa）。当在生理条件下进行凝胶色谱分析时，TL抗原主要由大小与免疫球蛋白G相似的分子组成。通过间接免疫沉淀监测，在溶解的胸腺细胞膜大分子的蔗糖密度梯度超速离心中，TLa的单一成分很明显。TL抗原的沉降常数（5.8 S）远小于免疫球蛋白G的沉降常数。凝胶色谱和超速离心数据表明TLa的表观分子量约为120000。通过间接免疫沉淀分离的TLa由两种类型的多肽链组成。较小的亚基被鉴定为β2-微球蛋白。较大的多肽链携带同种异体抗原决定簇，还原和烷基化后分子量约为50000。在变性条件下进行分子量测定的TLa由两个组分组成。较小的组分是β2-微球蛋白，显然它仅通过非共价相互作用与较大的多肽链相连。较大的组分大小大于还原和烷基化的免疫球蛋白G重链。对后一组分进行还原和烷基化后，其大小减小，分子量似乎约为50000。因此，TLa由两条二硫键连接的重多肽链和两个β2-微球蛋白分子组成。通过木瓜蛋白酶消化溶解的TLa包含两条多肽链，其中一条是β2-微球蛋白。较大的37000道尔顿亚基是重多肽链的片段。用木瓜蛋白酶消化溶解的120000道尔顿TLa证明了这一点。获得的蛋白水解片段与通过蛋白水解直接从细胞表面释放的片段无法区分。木瓜蛋白酶衍生的TLa片段表现出了大部分（如果不是全部）同种异体抗原决定簇。