Lysophosphatidylcholine Acyltransferase 1 (LPCAT1) Specifically Interacts with Phospholipid Transfer Protein StarD10 to Facilitate Surfactant Phospholipid Trafficking in Alveolar Type II Cells

Anjaparavanda P. Naren,Sui Lin,Changsuk Moon,Machiko Ikegami,John M. Shannon

doi:10.1074/jbc.m115.666701

Abstract

Pulmonary surfactant, a mixture of proteins and phospholipids, plays an important role in facilitating gas exchange by maintaining alveolar stability. Saturated phosphatidylcholine (SatPC), the major component of surfactant, is synthesized both de novo and by the remodeling of unsaturated phosphatidylcholine (PC) by lyso-PC acyltransferase 1 (LPCAT1). After synthesis in the endoplasmic reticulum, SatPC is routed to lamellar bodies (LBs) for storage prior to secretion. The mechanism by which SatPC is transported to LB is not understood. The specificity of LPCAT1 for lyso-PC as an acyl acceptor suggests that formation of SatPC via LPCAT1 reacylation is a final step in SatPC synthesis prior to transport. We hypothesized that LPCAT1 forms a transient complex with SatPC and specific phospholipid transport protein(s) to initiate trafficking of SatPC from the endoplasmic reticulum to the LB. Herein we have assessed the ability of different StarD proteins to interact with LPCAT1. We found that LPCAT1 interacts with StarD10, that this interaction is direct, and that amino acids 79-271 of LPCAT1 and the steroidogenic acute regulatory protein-related lipid transfer (START) domain of START domain-containing protein 10 (StarD10) are sufficient for this interaction. The role of StarD10 in trafficking of phospholipid to LB was confirmed by the observation that knockdown of StarD10 significantly reduced transport of phospholipid to LB. LPCAT1 also interacted with one isoform of StarD7 but showed no interaction with StarD2/PC transfer protein.

Highlights

Surfactant phosphatidylcholine (PC) must be transported to lamellar bodies (LBs) prior to secretion from type II cells
Most strikingly, when we compared expression of the different START domain-containing (StarD) proteins in adult type II cells isolated by FACS from the lungs of surfactant protein C/GFP mice, we found that START domain-containing protein 10 (StarD10) mRNA was 31 times more abundant than StarD2/PCTP and 8 times more abundant than StarD7 (Fig. 1E; p Ͻ 0.0001, n ϭ 3)
Surfactant phospholipids are synthesized in the endoplasmic reticulum (ER) and transported to mature LB where they are stored before secretion, but the mechanism by which this occurs is currently unknown

Summary

Background

Surfactant phosphatidylcholine (PC) must be transported to lamellar bodies (LBs) prior to secretion from type II cells. Results: The acyltransferase LPCAT1 interacts with the phospholipid transfer protein StarD10 in vivo and in vitro. We hypothesized that LPCAT1 forms a transient complex with SatPC and specific phospholipid transport protein(s) to initiate trafficking of SatPC from the endoplasmic reticulum to the LB. Pulmonary surfactant is a complex mixture of phospholipids and proteins that is synthesized, stored, and secreted by alveolar type II epithelial cells in the lung [1]. Because reacylation of lyso-PC is a final step in SatPC synthesis prior to its trafficking to LB, we hypothesized that LPCAT1 forms a transient complex with SatPC and a specific phospholipid transport protein(s) to initiate the movement of SatPC from the ER to the LB. In concert with previous data [19, 44], these results provide further evidence that LPCAT1 is a multifunctional enzyme that plays important roles in diverse cellular functions

Experimental Procedures

Results

Discussion