Molecular Basis of Lysosomal Enzyme Recognition: Three-Dimensional Structure of the Cation-Dependent Mannose 6-Phosphate Receptor

Abstract

Targeting of newly synthesized lysosomal hydrolases to the lysosome is mediated by the cation-dependent mannose 6-phosphate receptor (CD-MPR) and the insulin-like growth factor II/cation-independent mannose 6-phosphate receptor (IGF-II/CI-MPR). The two receptors, which share sequence similarities, constitute the P-type family of animal lectins. We now report the three-dimensional structure of a glycosylation-deficient, yet fully functional form of the extracytoplasmic domain of the bovine CD-MPR (residues 3-154) complexed with mannose 6-phosphate at 1.8 A resolution. The extracytoplasmic domain of the CD-MPR crystallizes as a dimer, and each monomer folds into a nine-stranded flattened beta barrel, which bears a striking resemblance to avidin. The distance of 40 A between the two ligand-binding sites of the dimer provides a structural basis for the observed differences in binding affinity exhibited by the CD-MPR toward various lysosomal enzymes.

M6P/IGF2R is mutated in squamous cell carcinoma of the lung

“…The third mutation detected in repeat domain 11 of the receptor is a G:C?A:T transition at a CpG site in exon 34 that results in the substitution of Arg for Gly1619 (Figure 2e,f). This amino acid alteration is also predicted from the 3-D structure of the CDM6PR to reside in a putative loop region between beta strands 7 and 8 (Roberts et al, 1998). The substitution of Arg, a large charged amino acid, for Gly1619, a small neutral amino acid, would be expected to signi®cantly alter receptor tertiary structure and function.…”

“…This amino acid change is predicted from the 3-D structure of the CDM6PR to reside in a putative loop region between beta strands 7 and 8 (Roberts et al, 1998). The substitution of Thr, an uncharged polar amino acid, for Ala1618, a nonpolar hydrophobic amino acid, would be expected to a ect the receptor tertiary structure and function since the interactions between the two orthogonal beta sheets is principally hydrophobic in nature (Roberts et al, 1998). The hydrophobic interactions between the beta sheets makes the overall structure very tight and compact.…”

“…A G:C?C:G transversion was identi®ed at a non CpG site in exon 33 that results in the substitution of Arg for Gly1564 (Figure 2c). This amino acid alteration occurs within the identi®ed minimal IGF2 binding site (Dahms et al, 1994;Garmroudi et al, 1996;Schmidt et al, 1995), and is predicted from the 3-D structure of the CDM6PR to be in a putative loop region between beta strands 2 and 3 (Roberts et al, 1998). The substitution of Arg, a large charged amino acid, for Gly1564, a small neutral amino acid, would be expected to signi®cantly alter receptor tertiary structure and function.…”

“…This amino acid is conserved among human, bovine, rat, mouse and chicken indicating that Gly1296 is important for receptor function. The secondary structure of this repeat region has been modeled on the 3-D structure of the CDM6PR which forms a nine stranded¯attened b barrel (Roberts et al, 1998). Based upon this predicted secondary structure, Gly1296 of repeat 9 resides in the loop region between beta strands 4 and 5, the transition region between the two orthogonal b sheets.…”

“…b Based on Morgan et al (1987). c Based on the 3D structure of the CDM6PR (Roberts et al, 1998) Oncogene M6P/IGF2R mutation in lung cancer F-M Kong et al majority of tumors with LOH at the M6P/IGF2R locus (10/11) lack detectable receptor protein even though the remaining allele is not mutated in the receptor binding regions. This could result from the allele being mutated in gene regions not screened in this investigation.…”