Concluding Step in Cell Restitution Cycle: ER Transport Vesicles with Sphingolipids in the Outer Leaflet of the Membrane Restore Lysosomes

Abstract

Restitution of the cell organelles and the membrane implicates serine palmitoyltransferase (SPT) in signal-specific and selective assembly of the transport vesicles. Here, we reveal that SPT, embedded in the outer leaflet (OL) of endoplasmic reticulum (ER), is engaged in the synthesis of ER transport vesicles that recondition cell organelles, and the inner leaflet (IL) SPT in the restitution of the cell membrane. The OL SPT impacts assembly of sphingomyelinase (SMase)-susceptible ER vesicles but not the SMase-resistant and sphingolipid (SPhL) core-carrying vesicles that refurbish the cell membrane. The investigation of the SPT-initiated differences in the placement of SPhL in vesicular membranes by utilizing ER depleted of OL SPT, allows us to conclude that the restitution of endosomal and lysosomal membranes is achieved with the involvement of OL SPT, whereas the IL SPT is involved in formation of the lipid core for glycosphingolipids (GSL) and sphingomyelin (SM) of the apical and basolateral cell membrane. These findings along with our previously published report (Slomiany and Slomiany, Advances in Biological Chemistry, 2013, 3, 275-287), provide a clear distinction between the processes that renovate cell membrane and its organelles from that of the endocytotic cell debridement, and show that vesicles are navigated to the specific organelles and the cell membrane by the biomembrane constituents programmed in ER.

Mitochondrial Membranes Restitution Proceeds via Vesicular Import from ER and Cytosol. Counterparts’ Resemblances and Variances in Mitochondria and Golgi Pathways

“…Based on the paradigm used, and the evidence obtained from the investigations of Golgi-directed transport vesicles that represent assortment of specific products that restore Golgi, endosome and cell membrane [2]- [7], henceforth, we speculated that mitochondria-directed transporters must consist of the vesicles assemblies dedicated for the restoration of OMM, IMM and, that IMMbound transporters carry cytosolic proteins for mitochondrial matrix. Moreover, as our concept and the results dictated, the synthesis of OMM and IMM-directed transport vesicles although initially displaying same lipid composition must be initiated through different signals since their protein composition is not identical, the IMM is richer in protein than OMM and is enriched in CL [5].…”

“…This poses fastidious tasks on trafficking of the newly synthesized replacements to their exact sites and especially presents incredibly difficult challenge for the import of genome encoded ER-and cytosol-residing components to mitochondria [3] [5] [6] [7] [10] [11] [12]. Consequently, numerous pathways for protein trafficking and translocation across the organelle's membranes are described [10] [13]- [24].…”

“…Consequently, numerous pathways for protein trafficking and translocation across the organelle's membranes are described [10] [13]- [24]. Yet, the molecular mechanisms that distinguish different pathways still require deeper exploration, and above all, the investigation of the role of lipids in controlling and discerning these processes [2]- [7] [10] [25] [26] [27].…”

“…The criteria that satisfy the requirements of membrane make up fidelity and the deposition of organelle specific proteins were systematically investigated in our work [1]- [7] [45]. Coherent picture derived from the multiplicity of experiments revealed that the only process that fulfills these principle requirements is ER-localized biogenesis of the genomically-defined membrane proteins cotranslationally intercalated within explicit set of co-synthesized lipids that together form a distinct, organelle-precise components in a form of transport vesicles.…”

“…We have demonstrated that the mitochondria-specific ER-assembled transport vesicles are formed and are substantially dissimilar from those generated for restitution of Golgi, endosomes and cell membrane [3] [5] [7]. The major differences in membrane lipid composition found in vesicles destined to mitochondria is the presence of phosphatidylglycerol (PG), the absence of sphingolipids (SphL) and phosphatidylinositol (PI), and that their fusion with OMM is accompanied by A. Slomiany, B. L. Slomiany generation of lysophosphatidylglycerol (LPG).…”