Summary
Proteins in the plasma membrane anchor the cell within its microenvironment and sense
changes occurring outside the cell. The anchoring interactions are cell type-specific
and may involve adjacent cells or extracellular matrix proteins (ECMPs). In development,
wound healing, and in various forms of pathology, including thrombosis and atherosclerosis,
the microenvironment of the cell may change rapidly and dramatically. How the cell
responds is strongly dependent on the protein composition of its plasma membrane,
which we refer to as the plasma membrane proteome. Processes that regulate the plasma
membrane proteome may alter cellular response. Low density lipoprotein receptor-related
protein-1 (LRP-1) is a member of the LDL receptor family; however, LRP-1 and other
less well studied members of this gene family demonstrate multiple activities unrelated
to lipid homeostasis. LRP-1 binds and internalizes numerous, structurally diverse
ligands, delivering most but not all these ligands to lysosomes for degradation. The
intracellular tail of LRP-1 binds signaling adaptor proteins and thus may function
in cell signaling. Biological activities of LRP-1 include antigen presentation, phagocytosis,
removal of apoptotic cells, and regulation of vascular permeability. This review focuses
on an emerging view of LRP-1 activity, in which LRP-1 regulates the protein composition
of the plasma membrane and thereby “models” or “landscapes” the cell surface. In some
cases, plasma membrane modeling results from the binding to bifunctional ligands or
intracellular adaptor proteins, so that LRP-1 is bridged to another plasma membrane
protein and the entire complex undergoes endocytosis. Membrane proteins already known
to be subject to this form of regulation include urokinase-type plasminogen activator
receptor, amyloid precursor protein, tissue factor, and αV-containing integrins. LRP-1 also controls the plasma membrane proteome by regulating
maturation and transport of proteins in the secretory pathway. At the same time, LRP-1
serves as a receptor for specific ECMPs, including fibronectin and thrombospondin.
Although ECMP-binding to LRP-1 results in endocytosis and catabolism, these receptor-ligation
events also may be coupled, directly or indirectly, to cell-signaling. Based on these
novel activities, LRP1 emerges as a protein capable of modeling the interface of the
cell with its microenvironment.
Keywords
Low density lipoprotein receptor-related protein - urokinasetype plasminogen activator
receptor - amyloid precursor protein - tissue factor - integrin - platelet-derived
growth factor receptor