Polyethylene glycol‐stimulated microsomal GTP hydrolysis

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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 (K m = 0.9 μM) GTPase. The effects of PEG reflect an increase in the V max of this activity; no effects on K m 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 GTPγS (K i = 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.