We previously purified lysophospholipase D (lysoPLD), which hydrolyzes lysophosphatidylcholine (lysoPC) to lysophosphatidic acid (LPA), from rat brain and identified the heterotrimeric G protein subunits G αq and G β1 in the lysoPLD active fractions. Tag-affinity purified G αq exhibits lysoPLD activity but a mutant that affected cellular localization or interaction with the G β subunit reduced lysoPLD activity. Size exclusion chromatography revealed that active lysoPLD is a much higher molecular weight complex than is heterotrimeric G protein, suggesting the presence of other components. Liquid chromatography-tandem mass spectrometry of lysoPLD purified from rat brain identified glycerophosphodiesterase 4 (GDE4), recently reported as lysoPLD, in the same fraction as G proteins. The over-expressed and tag-purified G αq fractions, which exhibit lysoPLD activity, contained GDE4. Exogenously expressed GDE4 was co-immunoprecipitated with endogenous G αq and G β and exhibited high lysoPLD activity. The results of confocal microscopy and cell fractionation experiments indicated that exogenously expressed GDE4 in cells mainly localized at the ER and partially co-localized with G αq protein at the plasma membrane. Proteinase K protection assay results suggested that the catalytic domain of GDE4 faces the lumen/extracellular space. Mutations at the conserved amino acids in the C-terminus cytoplasmic regions amongst GDE1, 4 and 7, dramatically suppressed GDE4 enzyme activities. When both the G αq and G α11 genes in Neuro2A cells were disrupted using the CRISPR-Cas9 system, endogenous lysoPLD activity was partially reduced but rescued by over-expression of G αq . These results suggest that GDE4 is a new effector of G protein signaling that produces bioactive phospholipid LPA and/or modulates membrane homeostasis.