Aster Proteins Facilitate Nonvesicular Plasma Membrane to ER Cholesterol Transport in Mammalian Cells

Abstract

The mechanisms underlying sterol transport in mammalian cells are poorly understood. In particular, how cholesterol internalized from HDL is made available to the cell for storage or modification is unknown. Here, we describe three ER-resident proteins (Aster-A, -B, -C) that bind cholesterol and facilitate its removal from the plasma membrane. The crystal structure of the central domain of Aster-A broadly resembles the sterol-binding fold of mammalian StARD proteins, but sequence differences in the Aster pocket result in a distinct mode of ligand binding. The Aster N-terminal GRAM domain binds phosphatidylserine and mediates Aster recruitment to plasma membrane-ER contact sites in response to cholesterol accumulation in the plasma membrane. Mice lacking Aster-B are deficient in adrenal cholesterol ester storage and steroidogenesis because of an inability to transport cholesterol from SR-BI to the ER. These findings identify a nonvesicular pathway for plasma membrane to ER sterol trafficking in mammals.

The cholesterol transfer protein GRAMD1A regulates autophagosome biogenesis

“…This data is consistent with the recently published report. 23 The PH-GRAM constructs of GRAMD1A-C cholesterol to the GRAMD1A StART domain could be inhibited by autogramin-2 in a dose-dependent manner, but not by the inactive analogue 8 (Figure 3d and Supplementary Figure 6e), suggesting that the autogramin binding site of GRAMD1A overlaps with the cholesterol binding site. Autogramin-2 did not compete with 22-NBD-cholesterol binding to GRAMD1B and C, further confirming its selectivity for GRAMD1A ( Figure 3d and Supplementary Figure 6e).…”

“…Recently, the StART domains of GRAMD1A-C were reported to transfer cholesterol 21 and X-ray structures of the GRAM domain of the GRAMD1A yeast homologue lipid transfer at contact site (Ltc1/Lam6) and the StART domains of Lam2 and 4 were reported 21,22 . Very recently the role of GRAMD1A-C (renamed Aster A-C) in sterol transfer between the ER and the plasma membrane has been reported 23 . However, no structures of human GRAMD1A-C have been reported and their roles in mediating membrane contacts, particularly in the context of autophagy, are unclear.…”

Sterol biosensor reveals LAM-family Ltc1-dependent sterol flow to endosomes upon Arp2/3 inhibition

“…Lam1-4 localize to ER-PM contact sites, with Lam2/Ysp2 involved in retrograde sterol transport (Gatta et al, 2015); Lam6/Ltc1 localizes to contact sites between ER, mitochondria, and vacuoles (Elbaz-Alon et al, 2015;Murley et al, 2015); Lam5/Ltc2 may localize to contact sites between the ER and the late Golgi (Weill et al, 2018). The mammalian homologues, called GramD1 or Aster, also localize to ER-PM contact sites and contribute to retrograde transport of exogenously provided cholesterol (Besprozvannaya et al, 2018;Sandhu et al, 2018). Intriguingly, while most LTPs localize to dedicated organelle contact sites, S. cerevisiae mutants lacking all six known ER-PM tethering proteins show normal anterograde and only partially diminished retrograde sterol transport (Quon et al, 2018).…”

Sterol flow between the plasma membrane and the endosome is regulated by the LAM family protein Ltc1

“…Lam1-4 localize to ER-PM contact sites, with Lam2/Ysp2 involved in retrograde sterol transport (Gatta et al, 2015); Lam6/Ltc1 localizes to contact sites between ER, mitochondria and vacuoles (Elbaz-Alon et al, 2015;Murley et al, 2015); Lam5/Ltc2 may localize to contact sites between the ER and the late Golgi (Weill et al, 2018). The mammalian homologues, variously called GramD1 or Aster, also localize to ER-PM contact sites and contribute to retrograde transport of exogenously provided cholesterol (Besprozvannaya et al, 2018;Sandhu et al, 2018).…”