cDNA Cloning and Functional Analysis of the p97 Subunit of the 26S Proteasome, a Polypeptide Identical to the Type‐1 Tumor‐Necrosis‐Factor‐Receptor‐Associated Protein‐2/55.11

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Abstract
Molecular cloning of cDNA for a new regulatory subunit, designated p97, of the human 26S proteasome showed that the polypeptide consists of 908 amino acid residues with a calculated molecular mass of 100 184 Da and an isoelectric point of 4.94. Computer analysis showed that p97 is very similar to type‐1 tumor‐necrosis‐factor(TNF)‐receptor‐associated protein (TRAP)‐2 and 55.11, both of which were identified recently as binding proteins of the cytoplasmic domain of type‐1 TNF receptor by yeast two‐hybrid screening. This finding suggests that the 26S proteasome might serve as a mediator molecule in the TNF signaling pathway in cells. Computer‐assisted similarity analysis also revealed the high sequence similarity of p97 with a yeast protein whose function is yet unknown, the gene for which is here termed NAS1 (non‐ATPase subunit 1). Disruption of NAS1 resulted in several phenotypes, including lethality and temperature‐sensitive growth, depending on the genetic background of the cells used. The human p97 cDNA suppressed the growth defect of nas1 disruptant cells, when expressed from single‐copy or multicopy vectors, indicating that p97 is functionally equivalent to yeast Nas1p. Culturing of the temperature‐sensitive nas1 cells at the restrictive temperature promoted the accumulation polyubiquitinated cellular proteins, implying that the 26S proteasome requires a functional Nas1p subunit for ubiquitin‐dependent proteolysis. These results indicate that p97/Nas1p plays an important regulatory role in the function of the 26S proteasome.