page_title: Oxysterol-binding protein
section_title: Main functions
hierarchical_section_title: Oxysterol-binding protein / Main functions
caption_attribution_description: English: They could function as lipid sensors that alter their interactions with other proteins in response to binding or releasing lipid ligands.
context_page_description: The oxysterol-binding protein-related proteins are a family of lipid transfer proteins. Concretely, they constitute a family of sterol and phosphoinositide binding and transfer proteins in eukaryotes that are conserved from yeast to humans. They are lipid-binding proteins implicated in many cellular processes related with oxysterol, including signaling, vesicular trafficking, lipid metabolism, and nonvesicular sterol transport.
  In yeast cells, some ORPs might function as sterol or lipid transporters though yeast strains lacking ORPs do not have significant defects in sterol transport between the endoplasmic reticulum and the plasma membrane. Although sterol transfer is proposed to occur at regions where organelle membranes are closely apposed, disruption of endoplasmic reticulum-plasma membrane contact sites do not have major effects on sterol transfer, though phospholipid homeostasis is perturbed. Various ORPs confine at membrane contacts sites, where endoplasmic reticulum is apposed with other organelle limiting membranes.
context_section_description: As part of the Lipid Transfer proteins (LTPs) family, ORPs have different and variate functions. This functions include signaling, vesicular trafficking, lipid metabolism and nonvesicular sterol transport. ORPs have been studied in many organisms cells as human cells or yeast. 
  In yeast, where organelle membranes are closely apposed it has been proposed that ORPs work as sterol transporters, though only a few ORPs actually bind sterols and collectively yeast ORPs are dispensable for sterol transfer in vivo. They are also part of Golgi-to-plasma membrane vesicular trafficking, but their role is not clear yet.
  In mammalian, ORPs participate as sterol sensors.This sensors regulate the assembly of protein complexes when cholesterol levels fluctuate.
   
  They use the following mechanisms:
  1-They could extract and deliver lipids from one membrane to another. Probably at membrane contact site.
  2-ORPs help establish the membrane when transient changes in the distribution of lipids occur. They add or remove lipids within different regions of the membrane. The exclusion of certain lipids in particular regions drive to processes such as membrane binding or signaling.
  3-They work as lipid sensors altering interactions with other proteins due to binding or releasing lipid ligands. It occurs mainly at inally organelle contact sites.
  4-The access of other lipid-binding proteins to the membrane is regulated by ORPs in two ways. One way is by presenting a lipid to a second lipid-binding protein. (5)Another way is preventing the lipid-binding protein from accessing a lipid in the membrane. This two mechanisms are not mutually exclusive so ORPs might use both.
caption: They could function as lipid sensors that alter their interactions with other proteins in response to binding or releasing lipid ligands.
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