HRF는 Na,K-ATPase의 ${alpha}$ subunit에 결합하여 이의 활성을 저해하는 것으로 알려져 있으며, PKC에 의해 Ser98 잔기가 인산화 될 수 있다는 것을 anti-HRFpS98 항체와 HRF S98A mutant를 이용한 실험으로 확인할 수 있었다. 또한 $^{86}Rb^{+}-uptake$ assay 실험에서 HRF의 serine 98 잔기의 탈인산화는 Na,K-ATPase의 활성에 약간의 영향을 미치는 것으로 미루어 PKC에 의해 인산화되는 98 serine 잔기가 Na,K-ATPase 활성 저해에 큰 영향을 미치지 않는 것으로 보인다.
IgE-dependent histamine-releasing factor (HRF) is found extracellularly to regulate the degranulation process of histamine in mast cells and basophils and known to play a predominant role in the pathogenesis of chronic allergic disease. HRF has been also identified in the intracellular region of the cell. Previously, we reported that HRF interacts with the 3rd cytoplasmic domain of the alpha subunit of Na,K ATPase and inhibits Na,K-ATPase activity. The predicated phosphorylation site in HRF by PKC was mapped to one serine residues (S98) by the computer analysis. In this study, we identified that S98 residue of HRF was phosphorylated using anti-HRFpS98 antibody which specifically recognizes the phosphorylated serine residue of HRF and HRFS98A mutant construct. We also performed $^{86}Rb^{+}-uptake$ assay to understand the role of HRF wild-type and HRFS98A mutants on the regulation of Na,K-ATPase activity. Dephosphorylation of HRF at serine 98 residue recovers slightly the inhibitory function of HRF, suggesting that phosphorylated HRF at serine 98 may not suppress the Na,K-hfpase activity.
