Crystal structure of the conserved domain of the DC lysosomal associated membrane protein: implications for the lysosomal glycocalyx.
BACKGROUND
The family of lysosome-associated membrane proteins (LAMP) comprises the multifunctional, ubiquitous LAMP-1 and LAMP-2, and the cell type-specific proteins DC-LAMP (LAMP-3), BAD-LAMP (UNC-46, C20orf103) and macrosialin (CD68). LAMPs have been implicated in a multitude of cellular processes, including phagocytosis, autophagy, lipid transport and aging. LAMP-2 isoform A acts as a receptor in chaperone-mediated autophagy. LAMP-2 deficiency causes the fatal Danon disease. The abundant proteins LAMP-1 and LAMP-2 are major constituents of the glycoconjugate coat present on the inside of the lysosomal membrane, the 'lysosomal glycocalyx'. The LAMP family is characterized by a conserved domain of 150 to 200 amino acids with two disulfide bonds.


RESULTS
The crystal structure of the conserved domain of human DC-LAMP was solved. It is the first high-resolution structure of a heavily glycosylated lysosomal membrane protein. The structure represents a novel β-prism fold formed by two β-sheets bent by β-bulges and connected by a disulfide bond. Flexible loops and a hydrophobic pocket represent possible sites of molecular interaction. Computational models of the glycosylated luminal regions of LAMP-1 and LAMP-2 indicate that the proteins adopt a compact conformation in close proximity to the lysosomal membrane. The models correspond to the thickness of the lysosomal glycoprotein coat of only 5 to 12 nm, according to electron microscopy.


CONCLUSIONS
The conserved luminal domain of lysosome-associated membrane proteins forms a previously unknown β-prism fold. Insights into the structure of the lysosomal glycoprotein coat were obtained by computational models of the LAMP-1 and LAMP-2 luminal regions.
Department of Molecular Structural Biology, Helmholtz Centre for Infection Research, Inhoffenstr, 7, 38124 Braunschweig, Germany.
Sonja
S
Wilke
Joern
J
Krausze
Konrad
K
Büssow
D000595:Amino Acid Sequence; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D019276:Glycocalyx / Q000378:metabolism*; D006031:Glycosylation; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D007425:Intracellular Membranes / Q000378:metabolism; D059666:Lysosomal-Associated Membrane Protein 3 / Q000737:chemistry* / Q000378:metabolism*; D008247:Lysosomes / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary
D059666:Lysosomal-Associated Membrane Protein 3