Membrane orientation of the precursor 74-kDa form of L-histidine decarboxylase

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Abstract.ObjectiveWe previously demonstrated that, when expressed in COS-7 cells, L-histidine decarboxylase (HDC), which has neither an amino terminal signal sequence nor a hydrophobic membrane anchor, was localized in the endoplasmic reticulum (ER), although its orientation in the membrane remains to be clarified.Methods & ResultsProtease digestion and immunofluorescence analyses of the cells, of which plasma membrane was selectively permeabilized, revealed that the amino terminal 50-kDa portion of HDC is hardly accessible to proteases and antibodies added exogenously from the cytosolic side. Green fluorescent protein fused with the carboxyl terminal 20-kDa region of HDC at its carboxyl terminus exhibited the same characteristics as native HDC.ConclusionThese results indicate that HDC is tightly associated with the ER membrane with its carboxyl terminal region exposed on the cytosolic side.

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