The Proteasome α-Subunit XAPC7 Interacts Specifically with Rab7 and Late Endosomes

Abstract

Rab7 is a key regulatory protein governing early to late endocytic membrane transport. In this study the proteasome ␣-subunit XAPC7 (also known as PSMA7, RC6 -1, and HSPC in mammals) was identified to interact specifically with Rab7 and was recruited to multivesicular late endosomes through this interaction. The protein interaction domains were localized to the C terminus of XAPC7 and the N terminus of Rab7. XAPC7 was not found on early or recycling endosomes, but could be recruited to recycling endosomes by expression of a Rab7-(1-174)Rab11-(160 -202) chimera, establishing a central role for Rab7 in the membrane recruitment of XAPC7. Although XAPC7 could be shown to associate with membranes bearing ubiquitinated cargo, overexpression had no impact on steady-state ubiquitinated protein levels. Most notably, overexpression of XAPC7 was found to impair late endocytic transport of two different membrane proteins, including EGFR known to be highly dependent on ubiquitination and proteasome activity for proper endocytic sorting and lysosomal transport. Decreased late endocytic transport caused by XAPC7 overexpression was partially rescued by coexpression of wild-type Rab7, suggesting a negative regulatory role for XAPC7. Nevertheless, Rab7 itself was not subject to XAPC7-dependent proteasomal degradation. Together the data establish the first direct molecular link between the endocytic trafficking and cytosolic degradative machineries.

β-Amyloid Accumulation Impairs Multivesicular Body Sorting by Inhibiting the Ubiquitin-Proteasome System

“…The mechanism by which this occurs appears unclear, because the proteasome is generally considered to be cytosolic and A␤ 42 accumulates in the inner side of, or within, the outer membrane of MVBs. However, A␤ 42 was reported to inhibit proteasome function in vitro (Gregori et al, 1995(Gregori et al, , 1997 and a direct interaction between the ␣-subunit XAPC7 of the proteasome to MVB/late endosomes was recently described (Dong et al, 2004). Moreover, by immunoelectron microscopy, we observed that the ␤-subunit of the 20S proteasome localizes to MVBs in neurons of the brain.…”

“…Localization of a proteasome subunit (XAPC7) to late endosomal membranes was recently described in BHK21 cells (Dong et al, 2004). To investigate whether proteasomes also localize to membrane compartments in neurons of the brain, we performed immuno-EM using an antibody against the 20S subunit of the proteasome (Fig.…”

Low Density Lipoprotein Receptor-related Protein 1 (LRP1) Controls Endocytosis and c-CBL-mediated Ubiquitination of the Platelet-derived Growth Factor Receptor β (PDGFRβ)

“…5, G and H). Recently, a new connection between the proteasome and membrane traffic has been established with the discovery that a proteasome ␣-subunit (XAPC7/PSMA7/HSPC) interacts with Rab7 on multivesicular bodies and that overexpression of this subunit interferes with the transport of membrane proteins from early to late endosomes (46). Exactly how proteasome inhibitors affect this interaction and thereby alter subcellular trafficking remains to be determined.…”

A Retromerlike Complex Is a Novel Rab7 Effector That Is Involved in the Transport of the Virulence Factor Cysteine Protease in the Enteric Protozoan ParasiteEntamoeba histolytica

“…Homology-based genome-wide search of the putative effectors of the amoebic Rab7A using known Rab7 effectors from other organisms, including RILP, Rabring7, hVps34p/p150 (phosphatidylinositol 3-kinase), and proteasome ␣-subunit (XAPC7) indicated that this organism possesses homologues for Vps34/p150 and XAPC7, but lacks RILP and Rabring7, suggestive of the presence of novel Rab7 effectors in this organism (Cantalupo et al, 2001;Mizuno et al, 2003;Stein et al, 2003;Dong et al, 2004). Thus, we attempted to isolate the effector molecules of EhRab7A by affinity purification to gain an insight in the regulation of the amoebic Rab7A.…”