Are you sure that these GTP analogues would cross the plasma membrane and enter your cells?

Apart from that in my experience the Kd of many enzymes for AMP-PNP is about

10, that for AMP-PCP about 100 times that for ATP (e.g., Biochem. J. 318

(1996) 923-929), if things are similar with GTP then you should reach 10-100

times the cytosolic concentration of GTP (500 µM) to see an effect.

@ARTICLE{Tra-94,

AUTHOR = {T.W. Traut},

TITLE = {Physiological concentrations of purines and pyrimidines},

YEAR = {1994},

JOURNAL = {Molecular and Cellular Biochemistry},

volume = {140},

url = {http://rasmodel.org/_downloads/7877593.pdf},

pages = {1-22},

}

This might constitute a logistic problem, as such nucleotid analogues are very expensive. You should also check them for contammination with GDP and GMP, which could give you misleading results.