Substrate access channel topology in membrane-bound prostacyclin synthase

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• Prostaglandin I(2) synthase (PGIS) is crucial for cardiovascular health.
• Previous studies suggested PGIS anchors to the endoplasmic reticulum (ER) membrane via its N-terminal segment.
• This anchoring mechanism was hypothesized to orient the substrate access channel towards the ER membrane.

Purpose of the Study:

• To experimentally validate the proposed topology model of PGIS.
• To determine the accessibility of the PGIS substrate channel opening in living cells.
• To investigate the orientation of the PGIS active site relative to the ER membrane.

Main Methods:

• Molecular modeling and site-directed mutagenesis were used to generate a PGIS model.
• Site-specific antibodies were generated against predicted surface peptides near the substrate channel opening.
• Immunofluorescence staining was performed on transfected and endogenous PGIS in cell lines (COS-1 and ECV cells).
• Antibody accessibility was assessed in cells with intact and permeabilized ER membranes.

Main Results:

• Four site-specific antibodies successfully recognized human PGIS.
• Antibodies targeting regions near the substrate channel (P66-75, P95-116) only bound after cell membrane permeabilization.
• Antibodies targeting regions further from the channel (P366-382, P472-482) bound to PGIS in cells with intact ER membranes.
• These differential binding patterns indicate the substrate channel opening is oriented towards the ER membrane.

Conclusions:

• The study provides experimental evidence supporting the PGIS topology model.
• The PGIS substrate access channel opening is localized close to the ER membrane.
• This orientation is critical for PGIS function within the ER.