POMT1 (Protein O-Mannosyltransferase 1) is an endoplasmic reticulum (ER) membrane-bound enzyme that catalyzes the first step in the O-mannosylation of proteins, particularly α-dystroglycan (α-DG). This post-translational modification is essential for the proper function of dystroglycan at the cell membrane, which serves as a critical link between the extracellular matrix and the cytoskeleton. Mutations in POMT1 cause severe forms of muscular dystrophy with brain involvement, including Walker-Warburg syndrome (WWS) and Limb-Girdle Muscular Dystrophy type 2K (LGMD2K) [@yoshidamoriguchi2010].
POMT1 (Protein O-Mannosyltransferase 1) is an endoplasmic reticulum (ER) membrane-bound enzyme that catalyzes the first step in the O-mannosylation of proteins, particularly α-dystroglycan (α-DG). This post-translational modification is essential for the proper function of dystroglycan at the cell membrane, which serves as a critical link between the extracellular matrix and the cytoskeleton. Mutations in POMT1 cause severe forms of muscular dystrophy with brain involvement, including Walker-Warburg syndrome (WWS) and Limb-Girdle Muscular Dystrophy type 2K (LGMD2K) [@yoshidamoriguchi2010].
Overview
POMT1 is a ~84 kDa protein encoded by the POMT1 gene located on chromosome 9q34.13. It belongs to the glycosyltransferase family 13 and is primarily localized to the endoplasmic reticulum, where it performs its enzymatic function. The protein catalyzes the transfer of mannose from the dolichol-phosphate-mannose donor to specific serine or threonine residues on target proteins, initiating the O-mannosylation pathway.
O-mannosylation is particularly important in muscle and brain tissue, where functional α-dystroglycan is essential for maintaining the structural integrity of muscle fibers and for proper neuronal migration and brain development. The modification involves a cascade of enzymes, with POMT1 initiating the process and POMT2 (Protein O-Mannosyltransferase 2) forming a functional complex [@akasakakamanya2010].
Structure
POMT1 possesses the characteristic features of an ER-resident glycosyltransferase:
Domain Organization
N-terminal signal peptide - Targets the protein to the ER membrane
Multicopper oxidase-like domain - Predicted domain of unknown function
Multiple transmembrane regions - Mediates ER membrane association
catalytic domain - Contains the active site for mannose transfer
C-terminal ER retrieval signal - Maintains ER localization
The POMT1/POMT2 Complex
POMT1 functions as part of a heterodimeric complex with POMT2:
POMT1 provides the catalytic activity
POMT2 assists in substrate recognition
Both proteins are required for functional enzyme activity
Complex formation occurs in the ER lumen
Normal Function
POMT1 performs essential biochemical and cellular functions:
O-Mannosylation
The primary enzymatic function is O-mannosylation:
Catalyzes mannose transfer - First step in O-mannosylation pathway
Modifies α-dystroglycan - Primary substrate in muscle and brain
Produces phosphomannan - Creates the mannose-1-phosphate backbone
[Yoshida-Moriguchi T, et al. (2010). O-mannosylphosphorylation of alpha-dystroglycan in muscular dystrophy. Science. 330(6003):686-690](https://doi.org/10.1126/science.1181512)
[Martin PT. (2007). Congenital muscular dystrophies involve protein O-mannosyl glycosylation. J Child Neurol. 22(9):1097-1108](https://doi.org/10.1177/088307380702200909)
[Endo T. (2015). Glycobiology of alpha-dystroglycan and muscular dystrophy. J Biochem. 157(1):1-12](https://doi.org/10.1093/jb/mvu066)
[Muntoni F, et al. (2008). The neuronal involvement in Walker-Warburg syndrome. Brain. 131(Pt 9):2186-2195](https://doi.org/10.1093/brain/awn119)
[Akasaka-Manya K, et al. (2010). O-mannosylation in disease and development. J Biochem. 148(6):659-667](https://doi.org/10.1093/jb/mvq104)
[Akasaka-Manya K, et al. (2010). O-mannosylation in disease and development. J Biochem. 148(6):659-667](https://doi.org/10.1093/jb/mvq104)
[Lyalin D, et al. (2012). POMT1 and O-mannosylation in brain development. Hum Mol Genet. 21(R1):R153-R163](https://doi.org/10.1093/hmg/dds354)
[Martin PT. (2007). Congenital muscular dystrophies involve protein O-mannosyl glycosylation. J Child Neurol. 22(9):1097-1108](https://doi.org/10.1177/088307380702200909)
[Yoshida-Moriguchi T, et al. (2010). O-mannosylphosphorylation of alpha-dystroglycan in muscular dystrophy. Science. 330(6003):686-690](https://doi.org/10.1126/science.1181512)
[Endo T. (2015). Glycobiology of alpha-dystroglycan and muscular dystrophy. J Biochem. 157(1):1-12](https://doi.org/10.1093/jb/mvu066)
[Muntoni F, et al. (2008). The neuronal involvement in Walker-Warburg syndrome. Brain. 131(Pt 9):2186-2195](https://doi.org/10.1093/brain/awn119)
[Wells L. (2013). O-mannosylation and muscular dystrophy. J Cell Physiol. 228(12):2243-2250](https://doi.org/10.1002/jcp.24276)
[Jensen BS, et al. (2003). The O-mannosylation pathway: implications for disease and therapeutic targets. Drug Discov Today. 8(15):683-689](https://doi.org/10.1016/S1359-6446(03)02808-6)
[Beltr M, et al. (2014). POMT1 genotype-phenotype correlation in Walker-Warburg syndrome. Neuromuscul Disord. 24(9-10):707-712](https://doi.org/10.1016/j.nmd.2014.06.432)
[Schuster MK, et al. (2011). POMT1 expression and glycosylation in neural tissues. Glycobiology. 21(11):1395-1406](https://doi.org/10.1093/glycob/cwq158)