Hofstra H, Witholt B
J Biol Chem. 1985 Dec 15;260(29):16037-44.
We have investigated the assembly of the heat-labile enterotoxin (LT) subunits after their processing and segregation into the periplasmic space as mature LT A and LT B polypeptides. LT B starts associating into oligomers during or immediately after translocation through the cytoplasmic membrane. Binding to LT A occurs immediately after oligomerization. Over 80% of the LT B subunits have oligomerized, and over 50% have associated with LT A into holotoxin within 1 min after synthesis. The fate of newly synthesized LT A is totally different. There is an extensive overproduction of LT A relative to LT B and after membrane translocation it becomes part of a periplasmic pool of free LT A. It is then bound by LT B oligomers or degraded at such a rate that the free periplasmic LT A disappears from the pool with a half-time of 20-25 min. About half of the LT A is incorporated into holotoxin, while the other half is degraded. We conclude that LT subunits are translocated and processed in a ratio of about 2 A to 5 B. Since free LT A is either degraded slowly or bound to newly synthesized LT B oligomers, the net result is a steady state of 1.4 to 1.7 A subunits to 5 B subunits in the periplasm. About 60% of this LT A is bound by LT B to form periplasmic holotoxin with a subunit ratio of about 1 A to 5 B. The remaining 40% of periplasmic LT A occurs free.
我们研究了热不稳定肠毒素（LT）亚基在加工并分离到周质空间后作为成熟的LT A和LT B多肽的组装情况。LT B在通过细胞质膜转运期间或之后立即开始缔合成寡聚体。寡聚化后立即与LT A结合。合成后1分钟内，超过80%的LT B亚基已寡聚化，超过50%已与LT A缔合成全毒素。新合成的LT A的命运则完全不同。相对于LT B，LT A大量过量产生，膜转运后它成为游离LT A周质池的一部分。然后它被LT B寡聚体结合或以一定速率降解，使得游离的周质LT A以20 - 25分钟的半衰期从池中消失。约一半的LT A被整合到全毒素中，另一半则被降解。我们得出结论，LT亚基以约2个A与5个B的比例转运和加工。由于游离的LT A要么缓慢降解，要么与新合成的LT B寡聚体结合，最终结果是周质中A亚基与5个B亚基的稳态比例为1.4至1.7。约60%的这种LT A被LT B结合形成亚基比例约为1个A与5个B的周质全毒素。其余40%的周质LT A以游离形式存在。
