Transbilayer movement of phosphatidylserine in nonhuman erythrocytes: evidence that the aminophospholipid transporter is a ubiquitous membrane protein.

This research discusses the identification of a specific protein involved in moving phosphatidylserine, a type of lipid, within the cell membrane of various mammalian red blood cells. The study also explores the regulatory influences on this molecular action.

Identification of the Aminophospholipid Transporter

The scientists initially identified a certain protein within erythrocytes (red blood cells) that seemed to be critical for the movement of phosphatidylserine (PS), a lipid molecule that is an important component of cell membranes:

• The protein has been referred to as an ‘aminophospholipid transporter’, abbreviated as APT, due to its role in facilitating the movement of specific lipids through the cell membrane.
• The study mentioned by the authors – Connor & Schroit (1988) – posits that a protein of around 31-32 kDaltons (kDa), found within the erythrocytes, is most likely to be responsible for the movement of phosphatidylserine across the cell membrane’s layers.

Examination of APT in Other Mammalian Red Blood Cells

The researchers identified similar proteins in red blood cells from several different mammals:

• The proteins were found in the erythrocytes of bovines, equines, ovines, porcines, canines, caprines, and rhesus monkeys, all with the same approximate molecular weight (31-32 kDa). This shows the ubiquity of this protein across different species.
• Their function was similar to that found in human red blood cells, as they also could specifically transport a fluorescent PS analogue from the outer layer of the cell membrane to its inner leaflet.

Inhibition of PS Transport

The study also highlighted how the activity of these proteins could be controlled:

• Transport of PS could be reversibly inhibited with an inhibitor known as pyridyldithioethylamine (PDA), which is sulfhydryl-specific, hinting that sulfhydryl groups are crucial for the APT’s function.
• The same inhibitory effect of PDA was observed in human and murine cell lines, suggesting this process is a fundamental one across different cell types and organisms.

Characterization of the APT

Further analysis was conducted on these proteins, revealing more clues about their structure and position within the cell:

• Isolated plasma membranes from radiolabeled cells showed significant labeling of a 32,000-Da component, confirming the presence of the APT in the plasma membrane.
• Treatment of the cells with proteases, lectins, or antibodies did not inhibit PS transport, suggesting that the 32-kDa protein is integrated within the membrane, not on the surface, with no functional sites exposed to the cell exterior.