Characterization of PHRF1/KIAA1542, a PHD domain and RING finger protein

PHRF1/KIAA1542 is a protein of virtually unknown function containing a putative PHD domain and RING finger. Sequence variants in the PHRF1 gene have been uncovered in systemic lupus erythematosus (SLE). PHRF1 has been identified as a substrate of ATM and ATR, important kinases that modulate the cellular response to DNA damage. We hypothesize that this protein is an E3 ubiquitin ligase and may play a role in the DNA damage response. The aim of this study was to perform preliminary functional characterization of PHRF1, and to develop reagents and optimize experimental conditions for further study. Immunoblot analysis demonstrated that multiple isoforms of PHRF1 exist, most likely due to posttranslational modifications. Using both immunofluorescence microscopy and cellular fractionation, PHRF1 was found to be a predominantly nuclear protein that can be found either tightly associated with chromatin or in a soluble nuclear fraction. Additionally, PHRF1 assembles into nuclear foci following transcriptional inhibition but not following treatment with the DNA damaging agents ultraviolet irradiation or mitomycin C. Using an immunoprecipitation-mass spectrometry approach, PHRF1 was also found to interact with proteins involved in mRNA processing and translation