Ubiquitin Recognition and Chaperoning

Abstract

The yeast transcription factor SPT23 is a homolog of the mammalian transcription factor nuclear factor-κB (NF-κB). In its immature form, SPT23 is a 120-kD protein tethered to the membrane of the endoplasmic reticulum (ER). The processing of p120 seems to involve the following: (i) ubiquitination and proteolytic cleavage to a mature 90-kD (p90) form, and (ii) the function of a protein complex consisting of CDC4--a protein involved in the post-mitotic fusion of Golgi membranes--and Udf1 and Npl4, which are thought to be involved in ubiquitin-dependent protein degradation and nuclear transport, respectively. However, the mechanism of SPT23 activation is largely unresolved. Rape et al. have found that p120 SPT23 must homodimerize at the ER membrane, as the first step in maturation. Yeast two-hybrid assays showed that the minimal region that conferred homodimerization was the immunoglobulin (Ig)-like-plexins-transcription factors (IPT) domain, and that IPT domains could associate with each other, suggesting that an IPT-IPT interaction was necessary and sufficient for SPT23 homodimerization. One of the SPT23 monomers became ubiquitinated by the ubiquitin ligase RSP5. The proteasome proteolytically cleaved the stalk that tethers SPT23 to the ER membrane. However, the cleaved SPT23, now p90, remained stably associated with the unprocessed monomer, as demonstrated by washing with a gradient of low-to-high salt concentrations. The removal of mature p90 from the immature monomer required the ATP-dependent activity of CDC48, which could discriminate the ubiquitinated p90 from p120. Additionally, the translocation of p90 to the nucleus was also CDC48-dependent, suggesting that CDC48 may act as a ubiquitin-selective chaperone protein. M. Rape, T. Hoppe, I. Gorr, M. Kalocay, H. Richly, S. Jentsch, Mobilization of processed, membrane-tethered SPT23 transcription factor by CDC48 UFD1/NPL4 , a ubiquitin-selective chaperone. Cell 107 , 667-677 (2001). [Online Journal]