Stimulation of glycosylphosphatidylinositol biosynthesis in mammalian cell-free systems by GTP hydrolysis: evidence for the involvement of membrane fusion
Vl. Stevens et al., Stimulation of glycosylphosphatidylinositol biosynthesis in mammalian cell-free systems by GTP hydrolysis: evidence for the involvement of membrane fusion, BIOCHEM J, 341, 1999, pp. 577-584
The second step in glycosylphosphatidylinositol (GPI) biosynthesis, the dea
cetylation of GlcNAc-phosphatidylinositol (GlcNAc-PI), has been shown to be
stimulated by GTP hydrolysis [Stevens (1993) J. Biol. Chem. 268, 9718-9724
]. We have now developed a system to study this regulation that uses micros
omes from cells defective in the first step in GPI biosynthesis (class A, C
and H lymphoma mutants) and the second reaction in the pathway (G9PLAP.85)
. With this mixed-microsome system, the deacetylation of GlcNAc-PI was almo
st completely dependent on GTP hydrolysis. Because GlcNAc-PI synthesized by
the G9PLAP.85 microsomes cannot readily move to the first-step-mutant micr
osomes to be deacetylated, this result indicated that the role of GTP was t
o facilitate the 'apparent' transfer of this substrate between membrane ves
icles. The microsomes could be stably preactivated by pretreatment with GTP
before GPI biosynthesis was initiated, indicating that fusion was the most
likely mechanism for this regulation. GlcNAc-PI deacetylation could also b
e stably preactivated in EL4 microsomes, suggesting that fusion also occurr
ed in wild-type membranes. Some differential localization of the GlcNAc-PI
synthetic and deacetylation activities with the endoplasmic reticulum was f
ound. Therefore fusion seems to stimulate GPI biosynthesis in mammalian mic
rosomes by bringing together the first two enzymes in the pathway in the sa
me membrane vesicle.