A splice variant of the human phosphohistidine phosphatase 1 (PHPT1) is degraded by the proteasome

Abstract

Regulation of protein activity by phosphorylation is central in many cellular processes. Phosphorylation of serine, threonine and tyrosine residues is well documented and studied. In addition, other amino acids, like histidine can be phosphorylated, but neither the mechanism nor the function of this modification is well understood. Nevertheless, there is a 14 kDa enzyme with phosphohistidine phosphatase activity, named PHPT1, found in most animals, but not in bacteria, plant or fungi. There are a few splice variant transcripts formed from the human PHPT1 locus and some of them are predicted to form variant proteins, but studies of these proteins are lacking. In order to get insight into the possible function of the variant transcripts encoded at the PHPT1 locus, ectopic expression of PHPT1 transcript variant 6, predicted to be degraded by the non-sense mediated mRNA decay pathway, in HeLa cells was undertaken. In HeLa cells the splice variant protein was degraded by the proteasome, unlike the wild type protein. Using an in silico modeling approach the variant C-terminal end of the proteins were predicted to form different secondary structures that might explain the different properties of the two proteins. The specific degradation of the PHPT1 splice variant indicates that at least for the PHPT1 protein, the quality control and the self-guarding of the cellular system works at two levels, first at the RNA level, aberrant transcripts are degraded by the non-sense mediated mRNA decay pathway, and the small amount of proteins that are formed will be degraded by the proteasome.

Molecular and cellular issues of KMT2A variants involved in Wiedemann-Steiner syndrome

“…4S). Reduction of KMT2A expression in fibroblasts bearing this splice variant, suggests that the mutated protein is likely degraded during its maturation [38].…”

A salt bridge of the C‐terminal carboxyl group regulates PHPT1 substrate affinity and catalytic activity

“…We speculate that the structural changes underlying the decrease in catalytic activity when the C-terminal salt bridge is disrupted could explain how alternative splicing could regulate PHPT1 function or the C-terminus could represent a binding site for a hypothetical effector that could downregulate PHPT1 activity if binding led to disruption of the salt bridge (Inturi et al, 2014).…”

“…Based on structural data, it has been proposed that the C-terminus forms a salt bridge with the sidechain guanidinium of R78 that may be important for organization of the active site (Gong et al, 2009). Interestingly, several isoforms of PHPT1 with significantly altered C-termini have been recently identified (Inturi et al, 2014). Given the potential importance of this region in the function of PHPT1, it is possible that PHPT1 activity may, in part, be regulated through alternative splicing that changes the identity of the C-terminus.…”