Moffett J, Englesberg E
Mol Cell Biol. 1984 Apr;4(4):799-808. doi: 10.1128/mcb.4.4.799-808.1984.
Chinese hamster ovary cells (CHO-K1) starved for 24 h for amino acids show a severalfold increase in velocity of proline transport through the A system (Vmax is five times that of unstarved cells). This increase is inhibited by cycloheximide, actinomycin D, N-methyl-alpha-amino isobutyric acid (MeAIB, a non-metabolizable specific A system amino acid analog), and by other amino acids that are generally transported by the A system. However, transport by the A system is not a prerequisite for this repression, and all compounds that have affinity for the A system do not necessarily act as "co-repressors." The addition of proline, MeAIB, or other amino acids, as described above, to derepressed cells results in a rapid decrease in A system activity. As shown with proline and MeAIB, this decrease in activity is in part due to a rapid trans-inhibition and a slow, irreversible inactivation of the A system. Neither process is inhibited by cycloheximide or actinomycin D. Alanine antagonizes the growth of CHO-K1 pro cells by preventing proline transport, and alanine-resistant mutants (alar) have been isolated (Moffett et al., Somatic Cell Genet. 9:189-213, 1983). alar2 and alar4 are partial and full constitutive mutants for the A system and have two and six times the Vmax for proline uptake by the A system, respectively. The A system in alar4 is also immune to the co-repressor-induced inactivation. Both alar2 and alar4 phenotypes are recessive. Alar3 shows an increase in Vmax and Km for proline transport through the A system, and this phenotype is codominant. All three mutants have a pleiotropic effect, producing increases in activity of the ASC and P systems of amino acid transport. This increase is not due to an increase in the Na+ gradient. The ASC and P phenotypes behave similarly to the A system in hybrids. A model has been proposed incorporating these results.
缺乏氨基酸24小时的中国仓鼠卵巢细胞（CHO-K1）通过A系统转运脯氨酸的速度增加了几倍（最大转运速度是未饥饿细胞的五倍）。这种增加受到环己酰亚胺、放线菌素D、N-甲基-α-氨基异丁酸（MeAIB，一种不可代谢的特异性A系统氨基酸类似物）以及通常由A系统转运的其他氨基酸的抑制。然而，通过A系统的转运并非这种抑制的先决条件，并且所有对A系统有亲和力的化合物不一定都作为“共抑制物”起作用。将脯氨酸、MeAIB或上述其他氨基酸添加到去阻遏细胞中会导致A系统活性迅速下降。如脯氨酸和MeAIB所示，这种活性下降部分是由于A系统的快速反式抑制和缓慢的、不可逆的失活。这两个过程均不受环己酰亚胺或放线菌素D的抑制。丙氨酸通过阻止脯氨酸转运来拮抗CHO-K1脯氨酸营养缺陷型细胞的生长，并且已经分离出丙氨酸抗性突变体（alar）（莫菲特等人，《体细胞遗传学》9:189 - 213，1983）。alar2和alar4分别是A系统的部分组成型和完全组成型突变体，通过A系统摄取脯氨酸的最大转运速度分别是正常细胞的两倍和六倍。alar4中的A系统对共抑制物诱导的失活也具有抗性。alar2和alar4的表型均为隐性。Alar3通过A系统转运脯氨酸的最大转运速度和米氏常数增加，并且这种表型是共显性的。所有这三个突变体都具有多效性作用，导致氨基酸转运的ASC和P系统活性增加。这种增加不是由于钠离子梯度的增加。在杂种中，ASC和P表型与A系统的表现相似。已经提出了一个包含这些结果的模型。
