Translocation of pyrene-labeled phosphatidylserine from the plasma membrane to mitochondria diminishes systematically with molecular hydrophobicity: implications on the maintenance of high phosphatidylserine content in the inner leaflet of the plasma membrane.

To study the translocation of phosphatidylserine (PS) from plasma membrane to mitochondria, dipyrene PS molecules (diPyr(n)PS; n=acyl chain length) were introduced to the plasma membrane of baby hamster kidney cells (BHK cells) using either cyclodextrin-mediated monomer transfer or fusion of cationic vesicles. Translocation of diPyr(n)PS to mitochondria was assessed based on decarboxylation by mitochondrial PS decarboxylase (PSD). It was found that the rate of translocation diminishes systematically with acyl chain length (molecular hydrophobicity) of diPyr(n)PS. Using an in vitro assay, it was shown that the spontaneous translocation rates of long-chain diPyr(n)PS species are similar to those of common natural PS species, thus supporting the biological relevance of the data. These results, and other data arguing against the involvement of vesicular traffic and lipid transfer proteins, imply that spontaneous monomeric diffusion via the cytoplasm is the main mechanism of PS movement from the plasma membrane to mitochondria. This finding could explain why a major fraction of PS synthesized by BHK cells consists of hydrophobic species: such species have little tendency to efflux from the plasma membrane to mitochondria where they would be decarboxylated. Thus, adequate molecular hydrophobicity seems to be crucial for the maintenance of high PS content in the inner leaflet of the plasma membrane.