Structure of mouse Golgi alpha-mannosidase IA reveals the molecular basis for substrate specificity among class 1 (family 47 glycosylhydrolase) alpha1,2-mannosidases.
Three subfamilies of mammalian Class 1 processing alpha1,2-mannosidases (family 47 glycosidases) play critical roles in the maturation of Asn-linked glycoproteins in the endoplasmic reticulum (ER) and Golgi complex as well as influencing the timing and recognition for disposal of terminally unfolded proteins by ER-associated degradation. In an effort to define the structural basis for substrate recognition among Class 1 mannosidases, we have crystallized murine Golgi mannosidase IA (space group P2(1)2(1)2(1)), and the structure was solved to 1.5-A resolution by molecular replacement. The enzyme assumes an (alphaalpha)(7) barrel structure with a Ca(2+) ion coordinated at the base of the barrel similar to other Class 1 mannosidases. Critical residues within the barrel structure that coordinate the Ca(2+) ion or presumably bind and catalyze the hydrolysis of the glycone are also highly conserved. A Man(6)GlcNAc(2) oligosaccharide attached to Asn(515) in the murine enzyme was found to extend into the active site of an adjoining protein unit in the crystal lattice in a presumed enzyme-product complex. In contrast to an analogous complex previously isolated for Saccharomyces cerevisiae ER mannosidase I, the oligosaccharide in the active site of the murine Golgi enzyme assumes a different conformation to present an alternate oligosaccharide branch into the active site pocket. A comparison of the observed protein-carbohydrate interactions for the murine Golgi enzyme with the binding cleft topologies of the other family 47 glycosidases provides a framework for understanding the structural basis for substrate recognition among this class of enzymes.
Tempel
Wolfram
W
Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia 30602, USA.
Karaveg
Khanita
K
Liu
Zhi-Jie
ZJ
Rose
John
J
Wang
Bi-Cheng
BC
Moremen
Kelley W
KW
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002236:Carbohydrate Conformation; D002240:Carbohydrate Sequence; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006056:Golgi Apparatus / Q000201:enzymology*; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D015394:Molecular Structure; D009844:Oligosaccharides / Q000737:chemistry / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D016415:Sequence Alignment; D013379:Substrate Specificity; D043323:alpha-Mannosidase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
D009844:Oligosaccharides; D043323:alpha-Mannosidase