Table 1.

Activity and expression of GlyT1b mutations at Ser399 and Gln299

Glyt1b	Glycine influx at 150 mM Cl−, % of WT (or S244C-C62A)	Glycine influx without Cl−, % of 150 mM Cl−	Cell surface expression, % of WT	Normalized transport activity, % of WT
WT	100	6 ± 1	100	100
S339D	14.8 ± 1.9***	88 ± 3**	22 ± 4**	68 ± 20*
S339E	15.2 ± 0.7***	85 ± 6**	34 ± 5**	44 ± 13**
Q299G	13.3 ± 1.1***	nd	89 ± 9	15 ± 3.4**
Q299N	11.9 ± 0.8***	nd	73 ± 6*	16 ± 2.9**
Q299E	4.2 ± 2.6***	nd	67 ± 4*	6.3 ± 4.1**
Q299K	2.3 ± 1.7***	nd	70 ± 8*	3.2 ± 2.6**

For WT and Ser339 mutants, [³H]glycine (50 nM) influx was measured in both Hepes buffered saline buffer containing 150 mM NaCl and Na-gluconate as described in Materials and Methods. Activity and surface expression levels for Gln299 mutants are relative to the S244C-C62A background construct, in which Gln299 mutants were generated. Cell surface expression was determined by biotinylation as described in Materials and Methods. Normalized transport activity shows the influx rate corrected for the surface expression level, relative to WT or S244C-C62A. The values represent means ± SEM; n = 3 for glycine influx; n = 4 for cell surface expression. Asterisks indicate significant differences from WT (*P < 0.05, **P < 0.01, ***P < 0.001) using Student’s t test. nd, not determined.