Transmembrane signaling: tumor promoter distribution

Area of Science:

• Cell biology
• Biochemistry
• Molecular pharmacology

Background:

• Diacylglycerol activates protein kinase C (PKC) in transmembrane signaling.
• The tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA) mimics diacylglycerol, activating PKC and phosphorylating membrane proteins in human erythrocytes.
• The precise location of TPA within plasma membranes is not well understood, hindering molecular modeling.

Purpose of the Study:

• To determine the transbilayer distribution of TPA within the human red cell plasma membrane.
• To provide direct evidence for TPA's localization to inform its mechanism of action.

Main Methods:

• Utilized membrane-splitting techniques based on freeze-fracture of planar cell monolayers.
• Quantified the transmembrane partitioning of radiolabeled [3H]TPA.
• Assessed TPA association with the red cell plasma membrane under conditions of stimulated PKC-mediated phosphorylation.

Main Results:

• Over 90% of [3H]TPA associated with the human red cell plasma membrane.
• Following bilayer splitting, two-thirds of the TPA partitioned to the cytoplasmic leaflet.
• This represents the first direct topographic localization of TPA in a biological membrane.

Conclusions:

• TPA preferentially associates with the cytoplasmic leaflet of the plasma membrane.
• This localization supports the hypothesis that TPA activation of PKC requires its presence on the cytoplasmic side of the membrane.