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  • Title: Polyethylene glycol-stimulated microsomal GTP hydrolysis. Relationship to GTP-mediated Ca2+ release.
    Author: Nicchitta CV, Joseph SK, Williamson JR.
    Journal: FEBS Lett; 1986 Dec 15; 209(2):243-8. PubMed ID: 3025017.
    Abstract:
    It has recently been observed that GTP mediates Ca2+ release from internal Ca2+ stores. In contrast to effects on permeabilized cells, GTP-dependent Ca2+ release in isolated microsomes requires the presence of polyethylene glycol (PEG). We have investigated the effects of PEG on microsomal GTPase activity and report that PEG stimulates a high-affinity (Km = 0.9 microM) GTPase. The effects of PEG reflect an increase in the Vmax of this activity; no effects on Km were observed. The concentration dependence for PEG-dependent stimulation of the high-affinity GTPase exactly mimicked that for GTP-dependent Ca2+ release. The stimulation of GTP hydrolysis by PEG was specific for the microsome fraction; only small effects were obtained with plasma membrane or cytosol fractions. As observed for GTP-dependent Ca2+ release, the microsomal PEG-stimulated GTPase was competitively inhibited by the GTP analog GTP gamma S (Ki = 60 nM). It is proposed that the PEG-stimulated GTPase may represent an intrinsic activity of the guanine nucleotide binding protein involved in the regulation of reticular Ca2+ fluxes.
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