Kinase Family POMK
POMK (Protein O-Mannose Kinase) is a poorly studied kinase family, reported to phosphorylate O-mannose as part of protein glycosylation. It was originally published as SgK196 (Sugen Kinase 196).
POMK is found in holozoans but it lost from several lineages, including insects and nematodes. It is a single copy gene in all vertebrates.
The protein is composed mostly of an ePK kinase domain with a moderate N-term extension. No clear signal peptide is seen but most homologs have a predicted transmembrane region (TM-HMM) just upstream of the kinase domain. The kinase domain contains four highly conserved cysteine residues within the kinase domain, another two close to its N-terminal border and a further conserved Cys just upstream. Similar to the RESK kinases, these conserved cysteines are unusual in kinases and may indicate presence in the Golgi or ER. Several of the canonical kinase motifs are divergent or unclear, and the gene was originally reported as a pseudokinase. The HRD motif lacks the Mg-coordinating N171 and the HRD is changed to MCD in most homologs.
POMK was found in a haploid genetic screen for defects in glycosylation of the ECM receptor α-dystroglycan . Biochemical analysis showed that POMK could phosphorylate protein-linked mannose (α-dystroglycan-O-Mannose), presumably in the Golgi apparatus . POMK may also be involved in post-translational modification of other O-mannose linked proteins including cadherins . Knockdown of the zebrafish homolog shows that it is required for muscle development , while mouse knockout  showed a hydrocephalus phenotype, both correlating with observed expression in muscle and brain.
Mutation of a conserved Q (Q258R) just after the APE motif and a conserved L (L137R)  were found in one patient with Walker-Warburg syndrome (WWS), a disease caused by failure to glycosylate α-dystroglycan. A stop mutation (Q109*) was found in a case of congenital muscular dystrophy (CMD), correlating with a lack of expression of α-dystroglycan protein .
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