Regulation of the A system of amino acid transport in Chinese hamster ovary cells, CHO-K1: the difference in specificity between the apo-repressor inactivator (apo-ri) and the transporter and the characterization of the proposed apo-ri.
When amino acids that are generally transported through the A system are added to derepressed cultures of CHO-K1 cells or to cultures that are undergoing starvation-derepression, as in the co-repressor (co-r), co-inactivator (co-i), (co-ri) assay, the A system undergoes trans-inhibition, inactivation, and repression. The effect of inactivation and repression is not related to the ability of amino acids to bind to the A system transporter but supports a model in which these amino acids act as co-r's/co-i's, and by binding to a aporepressor/inactivator (apo-ri), the product of gene R1, convert it into a repressor/inactivator (ri). For example, beta-alanine acts as a strong co-r but does not inhibit proline transport through the A system. Hydroxyproline and histidine, although poor inhibitors of proline transport, are very effective as co-ri's. Diaminobutyrate, phenylalanine, alpha-keto-glutarate, pyro-glutamate, isoleucine, and valine, compounds that inhibit A system transport, listed in decreasing order of effectiveness, are all equally poor as co-ri's. Also the Km for the transport of 2-(methylamino)isobutyric acid (MeAIB) through the A system is two times the concentration of MeAIB required to produce one-half inactivation. Amino acid effectors and mutation can modify the conversion of the apo-ri to repressor (r) and inactivator (i). The apo-ri is converted by alanine, serine, proline, and MeAIB to ri, by beta-alanine and tryptophane to r, and by hydroxyproline to r and reduced i. The full constitutive and partial constitutive mutants alar4 and alar2, respectively, are in the same complementation group. Alar4 has no active apo-ri while the rate of derepression of alar2 is twice and the inactivation rate is equal to that of the parent culture.