Fig. 6. Mutation of putative phosphorylation sites in the long GalT I cytoplasmic domain affects the expression of the GFP reporter on the cell surface. GFP fluorescence on processes of cells cultured on fibronectin was quantified as described in Materials and Methods. As expected, TS-GFP gave significantly lower surface fluorescence than did TL-GFP (P=0.0001). TL-GFP and mutations that change S11 or T18 to alanine (S11A, T18A and S11A,T18A) show the highest levels of surface GFP fluorescence, which are not significantly different from one another. In contrast, the S11D and T18D mutants both resulted in significantly decreased cell surface GFP fluorescence compared with TL-GFP. Mutating F3 and F7 to glycine (F3G,F7G) resulted in normal levels of surface GFP fluorescence. Data represents the mean ±s.e.m. from 3 to 6 individual determinations. P values were determined by Student's t test, and are shown for each construct. Using the Bonferroni method, statistical significance is calculated as P=0.05 divided by the number of mutants (in this case 6), therefore P<0.008 is statistically significant (asterisks).