Alanine-resistant mutants of Chinese hamster ovary cells, CHO-K1, producing increases in velocity of proline transport through the A, ASC, and P systems

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Abstract
We have developed a method for the isolation of transport mutants with increases in velocity of transport through the A and ASC systems and through a newly discovered P system utilizing the amino acid antagonism between A system amino acids and proline in CH0-K1 pro − cells. Mutants ala r 2and ala r 3,isolated in a single-step procedure, resistant to 25 mM alanine in MEM-10 plus 0.05 mM proline are pro −,stable, cross resistant to α-(methylamino)isobutyric acid (MeAIB) and show an approximately twofold increase in the initial velocity of proline uptake. Ethyl methane sulfonate (EMS) increases the frequency of pro − ala r clones in the population by at least 50 times the spontaneous frequency. The increased velocity of proline transport by ala r 2and ala r 3can be attributable to the 1.5 to 3 times increase in velocity of transport of proline through systems A, ASC, and P. The V max for proline transport through the A system has increased two times for ala r 2while the K m and V max for ala r 3has increased by 1.4 and 2.3 times that of CH0-K1. There is a corresponding increase in V max of proline transport by ala r 2through the P system. The P system is defined operationally as that portion of the Na +-dependent velocity that remains when the A, ASC, and glutamine-inhibitable fraction are eliminated. The system is concentrative. Proline appears to be the preferred substrate. Li + cannot be substituted for Na +.The system is moderately dependent upon pH. It obeys Michaelis-Menten kinetics and is not derepressible by starvation. There is no evidence for an N system in CHO-K1.