9.B.198 The Membrane-anchored Lipid-binding Protein (LAM) Family

Membrane contact sites are structures where two organelles come close together to regulate flow of material and information between them. One type of inter-organelle communication is lipid exchange, which must occur for membrane maintenance and in response to environmental and cellular stimuli. Soluble lipid transfer proteins have been extensively studied, but additional families of transfer proteins have been identified that are anchored into membranes by transmembrane helices. If such proteins target membrane contact sites, they may function in organellar lipid transfer. The eukaryotic family of so-called Lipid transfer proteins Anchored at Membrane contact sites (LAMs) all contain both a sterol-specific lipid transfer domain in the StARkin superfamily (related to StART/Bet_v1), and one or more transmembrane helices, anchoring them in the endoplasmic reticulum (ER). They target a variety of membrane contact sites, including contacts between organelles. Lam1-4p target punctate ER-plasma membrane contacts, while Lam5p and Lam6p target multiple contacts including vacuolar ER contacts. These developments confirm previous observations on tubular lipid-binding proteins (TULIPs) that establish the importance of membrane anchored proteins for lipid traffic (Wong and Levine 2016). However, it is not known if LAMs are transporters, or are regulators that affect traffic indirectly .


 

References:

Elbaz-Alon, Y., M. Eisenberg-Bord, V. Shinder, S.B. Stiller, E. Shimoni, N. Wiedemann, T. Geiger, and M. Schuldiner. (2015). Lam6 Regulates the Extent of Contacts between Organelles. Cell Rep 12: 7-14.

Gatta, A.T., L.H. Wong, Y.Y. Sere, D.M. Calderón-Noreña, S. Cockcroft, A.K. Menon, and T.P. Levine. (2015). A new family of StART domain proteins at membrane contact sites has a role in ER-PM sterol transport. Elife 4:.

Laraia, L., A. Friese, D.P. Corkery, G. Konstantinidis, N. Erwin, W. Hofer, H. Karatas, L. Klewer, A. Brockmeyer, M. Metz, B. Schölermann, M. Dwivedi, L. Li, P. Rios-Munoz, M. Köhn, R. Winter, I.R. Vetter, S. Ziegler, P. Janning, Y.W. Wu, and H. Waldmann. (2019). The cholesterol transfer protein GRAMD1A regulates autophagosome biogenesis. Nat Chem Biol 15: 710-720.

Murley, A., R.D. Sarsam, A. Toulmay, J. Yamada, W.A. Prinz, and J. Nunnari. (2015). Ltc1 is an ER-localized sterol transporter and a component of ER-mitochondria and ER-vacuole contacts. J. Cell Biol. 209: 539-548.

Naito, T. and Y. Saheki. (2021). GRAMD1-mediated accessible cholesterol sensing and transport. Biochim. Biophys. Acta. Mol. Cell Biol. Lipids 1866: 158957. [Epub: Ahead of Print]

Wong, L.H. and T.P. Levine. (2016). Lipid transfer proteins do their thing anchored at membrane contact sites… but what is their thing? Biochem Soc Trans 44: 517-527.

Examples:

TC#NameOrganismal TypeExample
9.B.198.1.1

LAM1 (YSP1) of 1228 aas and 1 or more TMSs (Wong and Levine 2016).

LAM1 of Saccharomyces cerevisiae

 
9.B.198.1.2

LAM3 (SIP3) of 1229 aas and 1 or more TMSs.

LAM3 of Saccharomyces cerevisiae

 
9.B.198.1.3

SipA3 of 482 aas and 0 TMSs

SipA of Verticillium alfalfae (Verticillium wilt of alfalfa) (Verticillium albo-atrum)

 
9.B.198.1.4

Uncharacterized protein of 1363 aas and 1 or more TMSs

UP of Rhizophagus irregularis (Arbuscular mycorrhizal fungus) (Glomus intraradices)

 
9.B.198.1.5

Uncharacterized protein of 1234 aas and 1 or more TMSs

UP of Mucor circinelloides (Mucormycosis agent) (Calyptromyces circinelloides)

 
Examples:

TC#NameOrganismal TypeExample
9.B.198.2.1

LAM2 (YSP2) of 1438 aas and 1 or more TMSs

LAM2 of Saccharomyces cerevisiae

 
9.B.198.2.2

LAM4 (LTC3) OF 1345 aas and 1 or more TMSs.

LAM4 of Saccharomyces cerevisiae

 
9.B.198.2.3

LAM5 (LTC2) of 674 aas and 1 or more TMSs

OF

LAM5 of Saccharomyces cerevisiae

 
9.B.198.2.4

Membrane-anchored lipid-binding protein, LAM6 or LTC1, of 693 aas and 1 (or more) TMSs (Wong and Levine 2016). It is involved in regulating various organellar membrane contact sites (Elbaz-Alon et al. 2015) and is involved in sterol transfer between intracellular membranes (Gatta et al. 2015). It selectively transports sterols between membranes in vivo and in vitro, and is involved in stress-dependent formation of sterol-enriched vacuolar membrane domains (Murley et al. 2015).

LAM6 of Saccharomyces cerevisiae

 
9.B.198.2.5

GRAM domain-containing protein 1B, GRAMD1B or ASTER-B, of 738 aas and 1 (or possibly more) TMSs. ASTER-A, B and C (TC#s 9.B.198.2.6, 5 and 7) can form homomeric and heteromeric complexes to move cholesterol and possibly phosphatidyl serine from the plasma membrane to the ER membrane (Naito and Saheki 2021).

GRAM domain protein 1B of Homo sapiens

 
9.B.198.2.6

GRAMD1A or Aster-A of 724 aas and 1 TMS near the C-terminus of the protein. It is a cholesterol transporter that mediates non-vesicular transport of cholesterol from the plasma membrane (PM) to the endoplasmic reticulum (ER) and  contains unique domains for binding cholesterol and the PM, thereby serving as a molecular bridge for the transfer of cholesterol from the PM to the ER. It plays a crucial role in cholesterol homeostasis and has the unique ability to localize to the PM based on the level of membrane cholesterol. In lipid-poor conditions, it localizes to the ER membrane, and in response to excess cholesterol in the PM is recruited to the endoplasmic reticulum-plasma membrane contact sites (EPCS) which is mediated by the GRAM domain. At the EPCS, the sterol-binding VASt/ASTER domain binds to the cholesterol in the PM and facilitates its transfer from the PM to ER. It may play a role in tumor progression, and plays a role in autophagy regulation while being required for biogenesis of the autophagosome which requires its cholesterol-transfer activity (Laraia et al. 2019).

GRAMD1A of Homo sapiens

 
9.B.198.2.7

GRAMD1C or Aster-C of 662 aas and 4 C-terminal TMSs. In lipid-poor conditions, it localizes to the ER membrane and in response to excess cholesterol in the plasma membrane (PM), it is recruited to the endoplasmic reticulum-plasma membrane contact sites (EPCS) which is mediated by the GRAM domain. At the EPCS, the sterol-binding VASt/ASTER domain binds to cholesterol in the PM and facilitates its transfer from the PM to ER.The GRAMDs form homo- and hetero-meric complexes to sense the levels of cholesterol in the PM and regulate transport of accessible PM cholesterol to the ER in order to maintain cholesterol homeostasis (Naito and Saheki 2021).

 

GRAMD1C of Homo sapiens