Structure, sequon recognition and mechanism of tryptophan C-mannosyltransferase
Joël S. Bloch, Alan John, Runyu Mao, Somnath Mukherjee, Jérémy Boilevin, Rossitza N. Irobalieva, Tamis Darbre, Nichollas E. Scott, Jean‐Louis Reymond, Anthony A. Kossiakoff, Ethan D. Goddard‐Borger, Kaspar P. Locher
Abstract
C-linked glycosylation is essential for the trafficking, folding and function of secretory and transmembrane proteins involved in cellular communication processes. The tryptophan C-mannosyltransferase (CMT) enzymes that install the modification attach a mannose to the first tryptophan of WxxW/C sequons in nascent polypeptide chains by an unknown mechanism. Here, we report cryogenic-electron microscopy structures of Caenorhabditis elegans CMT in four key states: apo, acceptor peptide-bound, donor-substrate analog-bound and as a trapped ternary complex with both peptide and a donor-substrate mimic bound. The structures indicate how the C-mannosylation sequon is recognized by this CMT and its paralogs, and how sequon binding triggers conformational activation of the donor substrate: a process relevant to all glycosyltransferase C superfamily enzymes. Our structural data further indicate that the CMTs adopt an unprecedented electrophilic aromatic substitution mechanism to enable the C-glycosylation of proteins. These results afford opportunities for understanding human disease and therapeutic targeting of specific CMT paralogs.