Entering the lysosome through a transient gate by chaperone-mediated autophagy

Abstract

A subset of cytosolic proteins can be selectively degraded in lysosomes through chaperone-mediated autophagy. The lysosomal-membrane protein type 2A (LAMP-2A) acts as the receptor for the substrates of chaperone-mediated autophagy (CMA), which should undergo unfolding before crossing the lysosomal membrane and reaching the lumen for degradation. Translocation of substrates is assisted by chaperones on both sides of the membrane, but the actual steps involved in this process and the characteristics of the translocation complex were, for the most part, unknown. We have now found that rather than a stable translocon at the lysosomal membrane, CMA substrates bind to monomers of LAMP-2A driving the organization of this protein into a high molecular weight multimeric complex that mediates translocation. Assembly and disassembly of LAMP-2A into and from this complex is dynamic and it is regulated by hsc70 and hsp90, the two lysosomal chaperones related to CMA. This work thus unveils a unique mechanism of protein translocation across the lysosomal membrane, which involves only transient discontinuity of the membrane. The possible advantages of this transitory lysosomal translocon are discussed in light of the unique properties of the lysosomal compartment.

Figure 1

CMA translocation at the lysosomal membrane. (A) Hypothetical model of the CMA translocation complex at the lysosomal membrane by analogy to the transport systems in other organelles. The translocation complex was conceived as a stable structure organized around multiple LAMP-2A (L-2A) molecules flanked by chaperones on both sides of the lysosomal membrane (L. Mb). Substrates bind to the exposed cytosolic tails, and chaperones facilitate substrate unfolding and translocation across the membrane and into the lysosomal matrix (L. Mtx). (B) Model of the CMA translocation complex proposed as a result of our recent studies. Substrate proteins bind to monomers of LAMP-2A, which progressively organize into a multimeric complex that facilitates delivery of substrate proteins to the lumen. Lysosomal chaperones, hsc70 and hsp90, interact with LAMP-2A when present in intermediate complexes formed during assembly and disassembly of the translocation complex. Other yet to be identified proteins (?) could also contribute to the formation of this translocation complex at the lysosomal membrane.