Chaperone-mediated autophagy and endosomal microautophagy: Joint by a chaperone

Abstract

A variety of mechanisms deliver cytosolic materials to the lysosomal compartment for degradation through autophagy. Here, we focus on two autophagic pathways, the chaperone-mediated autophagy and the endosomal microautophagy that rely on the cytosolic chaperone hsc70 for substrate targeting. Although hsc70 participates in the triage of proteins for degradation by different proteolytic systems, the common characteristic shared by these two forms of autophagy is that hsc70 binds directly to a specific five-amino acid motif in the cargo protein for its autophagic targeting. We summarize the current understanding of the molecular machineries behind each of these types of autophagy.

Keywords: autophagy; chaperone; lysosome; membrane protein; protein targeting; selective degradation.

Figure 1.

CMA and eMI in the context of mammalian autophagic pathways. In macroautophagy, cargo (proteins and organelles) sequestered inside autophagosomes in bulk (non selective macroautophagy) or selectively (left) is then delivered to lysosomes through autophagosome/lysosome fusion. In CMA all cargo (proteins) are selectively delivered to lysosomes upon recognition by hsc70 and targeting and binding to the lysosomal membrane protein LAMP2A. Microautophagy requires invagination of the lysosomal membrane to degrade cytosolic material. Proteins and organelles can also be targeted to late endosomes for degradation in mammals through what is known as endosomal microautophagy. Whether mammalian cells are able to directly invaginate the lysosomal membrane to trap cytosolic cargo, as described in yeast, remains unknown (??). CASA, chaperone-assisted selective autophagy.

Figure 2.

Steps and lysosomal membrane components of CMA. Proteins degraded through CMA are recognized by hsc70 in the cytosol (step 1) and are targeted to the lysosomal membrane where they bind to LAMP2A (step 2). Substrate binding triggers multimerization of LAMP2A (step 3) to form the complex that mediates substrate translocation (step 4). hsp90 stabilizes LAMP2A through this transition, and luminal hsc70 assists with the internalization of the substrate that then is rapidly degraded by lysosomal proteases (step 5). The stability of LAMP2A in the translocation complex is regulated by the depicted subset of proteins. Once substrate translocate, LAMP2A, dissociates into monomers (step 6). Changes in the turnover of LAMP2A at the lysosomal membrane also contribute to modulate CMA activity (step 7).

Figure 3.

Physiology of CMA. CMA participates in cellular quality control. CMA participates in (A) cellular quality control through the removal of damaged or abnormal proteins and in (B) cellular metabolism and energetics through recycling of the amino acids of the degraded proteins and by selectively degrading rate-limiting enzymes of lipid and glucose metabolism. C, timely degradation of specific proteins through CMA confers this pathway's regulatory function in multiple cellular processes. Some of these processes and the CMA substrates involved in these pathways are depicted.

Figure 4.

Mammalian endosomal microautophagy. Left, cytosolic proteins can be sequestered along with other cytosolic components by the invaginations that form in the surface of the endosomal membrane through the coordinated function of ESCRT (VPS4A/B and TSG101) and accessory proteins (Alix). Right, selective targeting to late endosomes of proteins bearing a KFERQ-like motif is mediated by hsc70. Upon cargo binding, hsc70 interacts directly with phosphatidylserine (PS) moieties of the endosomal membrane and is internalized along with the substrate in ESCRT-mediated microvesicles. Part of the internalized vesicles undergoes degradation in the endosomal lumen.

Figure 5.

Similarities and differences between CMA and eMI and assays to monitor protein degradation through them. A, summary of common and distinctive characteristics between CMA and eMI identified to date. B, recommended experimental approach to follow to discriminate involvement of CMA or eMI in the degradation of a protein. Blue boxes, methods; gray boxes, observed results.