We have identified a proteolytic activity in rat liver microsomes that specifically removes the C-terminus from a spectrum of ER chaperones. We refer to this activity as heat shock protein (hsp convertase (HSPC). All of the substrates for HSPC that we have identified are hsp, and contain the KDEL or KEEL at their C-termini, a signal sequence for ER retention. HSPC conversion of GRP94, ERp72 and calreticulin was rapid and no evidence of N-terminal alteration was detected. The conversion was unaffected by the presence of other membrane proteins. Two ER proteins that are very sensitive to non-specific proteases, cytochrome b5 and the NADH-cytochrome b5 reductase were also tested as substrates for the HSPC. SDS-PAGE and immunoblot analysis of the incubation mixture showed no alteration in the mobilities of the cytochrome b5 and its reductase. Lysomotropic agents leupeptin and pepstatin A were ineffective in inhibiting HSPC. The calpain inhibitor, N-acetyl-leucyl-leucyl-methional, or the teosome inhibitor lactacystin also failed to inhibit the HSPC activity. Specific enzymatic removal of the KDEL signal may represent a novel mechanism for the regulation of ER protein trafficking or the function of ER hsp. The discovery of HSPC may provide a biochemical explanation for observations that were previously attributed to the inefficiency of KDEL retention. Under special circumstances, hsp that are normally localized in the ER lumen are transported to the plasma membrane. The relocalization of ER hsps to the cell surface has been linked to malignant transformation and to apoptosis. ERp72 is expressed on the surface of human tumor cells, but is confined to the ER in normal cells. It is proposed that the physiological role of HSPC is to remove the ER retention signal from lumenal heat shock proteins thereby permitting the translocation of the modified chaperones into a variety of non-ER locales. Relocated, or modified chaperones may participate in cellular functions including protein degradation, antigen presentation, protein folding, cell adhesion, and the regulation of gene expression.
Heinemann et al. (Tue,) studied this question.