EPT1 (selenoprotein I) is critical for the neural development and maintenance of plasmalogen in humans

Phosphatidylserine (PS) and phosphatidylethanolamine (PE) are two aminophospholipids whose metabolism is interrelated. Both phospholipids are components of mammalian cell membranes and play important roles in biological processes such as apoptosis and cell signaling. PS is synthesized in mammalian cells by base-exchange reactions in which polar head groups of preexisting phospholipids are replaced by serine. PS synthase activity resides primarily on mitochondria-associated membranes and is encoded by two distinct genes. Studies in mice in which each gene has been individually disrupted are beginning to elucidate the importance of these two synthases for biological functions in intact animals. PE is made in mammalian cells by two completely independent major pathways. In one pathway, PS is converted into PE by the mitochondrial enzyme PS decarboxylase. In addition, PE is made via the CDP-ethanolamine pathway, in which the final reaction occurs on the endoplasmic reticulum and nuclear envelope. The relative importance of these two pathways of PE synthesis has been investigated in knockout mice. Elimination of either pathway is embryonically lethal, despite the normal activity of the other pathway. PE can also be generated from a base-exchange reaction and by the acylation of lyso-PE. Cellular levels of PS and PE are tightly regulated by the implementation of multiple compensatory mechanisms.

Plasmalogens (1-O-alk-1'-enyl-2-acyl glycerophospholipids) constitute a special class of phospholipids characterized by the presence of a vinyl-ether bond at the sn-1 position. Although long considered as biological peculiarities, interest in this group of phospholipids has grown in recent years, thanks to the realization that plasmalogens are involved in different human diseases. In this review, we summarize the current state of knowledge with respect to the enzymatic synthesis of plasmalogens, the characteristic topology of the enzymes involved and the biological roles that have been assigned to plasmalogens.

In yeast, Atg4/Apg4 is a unique cysteine protease responsible for the cleavage of the carboxyl terminus of Atg8/Apg8/Aut7, a reaction essential for its lipidation during the formation of autophagosomes. However, it is still unclear whether four human Atg4 homologues cleave the carboxyl termini of the three human Atg8 homologues, microtubule-associated protein light chain 3 (LC3), GABARAP, and GATE-16. Using a cell-free system, we found that HsAtg4B, one of the human Atg4 homologues, cleaves the carboxyl termini of these three Atg8 homologues. In contrast, the mutant HsAtg4B(C74A), in which a predicted active site Cys(74) was changed to Ala, lacked proteolytic activity, indicating that Cys(74) is essential for the cleavage activity of cysteine protease. Using phospholipase D, we showed that the modified forms of endogenous LC3 and GABARAP are lipidated and therefore were designated LC3-PL and GABARAP-PL. When purified glutathione S-transferase-tagged HsAtg4B was incubated in vitro with a membrane fraction enriched with endogenous LC3-PL and GABARAP-PL, the mobility of LC3-PL and GABARAP-PL was changed to those of the unmodified proteins. These mobility shifts were not seen when Cys(74) of HsAtg4B was changed to Ala. Overexpression of wild-type HsAtg4B decreased the amount of LC3-PL and GABARAP-PL and increased the amount of unmodified endogenous LC3 and GABARAP in HeLa cells. Expression of CFP-tagged HsAtg4B (CFP-HsAtg4B) and YFP-tagged LC3 in HeLa cells under starvation conditions resulted in a significant decrease in the punctate pattern of distribution of YFP-tagged LC3 and an increase in its cytoplasmic distribution. RNA interference of HsAtg4B increased the amount of LC3-PL in HEK293 cells. Taken together, these results suggest that HsAtg4B negatively regulates the localization of LC3 to a membrane compartment by delipidation.