STS. Fewer condensed nuclei were observed in EGFP PEST Hax 1 expressing cells than in EGFP Hax 1 expressing cells, suggesting that deletion of PEST sequence may increase Hax 1 stability, causing more resistance to STS induced apoptosis. Discussion Hax 1 transcript levels in mouse kidney, testis, and liver have previously been found to not directly correlate with detected protein levels. Similar phenomenon has also been observed in rat tissues. Two hypotheses to explain the different levels of mRNA compared to protein are that either high amounts of the Hax 1 tran script do not translate into proteins or that the protein degradation rate of Hax 1 is considerably high. Here, we provide clear evidence showing that Hax 1 protein is indeed turned over at a fast rate in a proteo some dependent manner.
It is important to note that, Hax 1 exists as many as 7 alternative splicing forms, and these splicing variants may play important roles in development or tumor formation. For example, the internal deletions in variants vII, vIV and vVI result in removal of BH domains and changes in PEST domain from variants I. It is therefore possible that these variant forms of Hax 1, because of its impair ment in PEST degradation signal, is more stable than its dominant form variant Carfilzomib I. The population of cells bearing an up regulation of these variants shows enhanced pro tective roles in tissues or more oncogenic activity, as evi denced in tumors. Polyubiquitination is required for the protein degrad ation by the proteasome.
Ubiquitin molecules, which form ubiquitin chains to a protein, are covalently linked to each other between a lysine site of the previous ubiquitin and the carboxy terminal glycine of a new ubiquitin. K48 linked polyubi quitination of a protein usually mediates its degradation by the proteasome, however, K63 linked polyubiquitina tion is most likely to play roles in translation, endocyto sis and other functions. In the present report, we demonstrate that Hax 1 is ubiquitinated via K48 linked ubiquitin chains. The ubiquitination of Hax 1 is largely dependent on its PEST sequence. In many short lived proteins, the PEST sequence serves as a signal se quence to drive their proteolysis or rapid degradation. In some cases, ubiquitination of proteins depends upon their PEST sequence. Here, we found that de letion of the PEST sequence results in much less ubi quitination of Hax 1, thereby increasing its stability.
It is therefore possible that the PEST sequence in Hax 1 is responsible for its proper folding to be conjugated with the ubiquitin chains. The PEST sequence is also reported to be a motif that is involved in protein modi fication. For example, phosphorylation of a PEST se quence by casein kinase II appears to promote the degradation of I��B. Also, a PEST like se quence has been shown to mediate phosphorylation and efficient ubiquitination of yeast uracil permease. Further studies to identify if the PEST sequence in Hax 1 is phosphorylated and if th