A molecular chaperone complex at the lysosomal membrane is required for protein translocation.

A group of cytosolic proteins are targeted to lysosomes for degradation in response to serum withdrawal or prolonged starvation by a process termed chaperone-mediated autophagy. In this proteolytic pathway little is known about how proteins are translocated across lysosomal membranes. We now show that an isoform of the constitutively expressed protein of the heat shock family of 70 kDa (Hsc70) is associated with the cytosolic side of the lysosomal membrane where it binds to substrates of this proteolytic pathway. Results from coimmunoprecipitation and colocalization studies indicate that this molecular chaperone forms complexes with other molecular chaperones and cochaperones, including Hsp90, Hsp40, the Hsp70-Hsp90 organizing protein (Hop), the Hsp70-interacting protein (Hip), and the Bcl2-associated athanogene 1 protein (BAG-1). Antibodies against Hip, Hop, Hsp40 and Hsc70 block transport of protein substrates into purified lysosomes.

Protein translocation across membranes.

Cellular membranes act as semipermeable barriers to ions and macromolecules. Specialized mechanisms of transport of proteins across membranes have been developed during evolution. There are common mechanistic themes among protein translocation systems in bacteria and in eukaryotic cells. Here we review current understanding of mechanisms of protein transport across the bacterial plasma membrane as well as across several organelle membranes of yeast and mammalian cells. We consider a variety of organelles including the endoplasmic reticulum, outer and inner membranes of mitochondria, outer, inner, and thylakoid membranes of chloroplasts, peroxisomes, and lysosomes. Several common principles are evident: (a) multiple pathways of protein translocation across membranes exist, (b) molecular chaperones are required in the cytosol, inside the organelle, and often within the organelle membrane, (c) ATP and/or GTP hydrolysis is required, (d) a proton-motive force across the membrane is often required, and (e) protein translocation occurs through gated, aqueous channels. There are exceptions to each of these common principles indicating that our knowledge of how proteins translocate across membranes is not yet complete.

Cytolytic T lymphocyte-associated antigen-4 and the TCR zeta/CD3 complex, but not CD28, interact with clathrin adaptor complexes AP-1 and AP-2.

The negative signaling receptor cytolytic T lymphocyte-associated Ag-4 (CTLA-4) resides primarily in intracellular compartments such as the Golgi apparatus of T cells. However, little is known regarding the molecular mechanisms that influence this accumulation. In this study, we demonstrate binding of the clathrin adaptor complex AP-1 with the GVYVKM motif of the cytoplasmic domain of CTLA-4. Binding occurred primarily in the Golgi compartment of T cells, unlike with AP-2 binding that occurs mostly with cell surface CTLA-4. Although evidence was not found to implicate AP-1 binding in the retention of CTLA-4 in the Golgi, AP-1 appears to play a role in shuttling of excess receptor from the Golgi to the lysosomal compartments for degradation. In support of this, increased CTLA-4 synthesis resulted in an increase in CTLA-4/AP-1 binding and a concomitant increase in the appearance of CTLA-4 in the lysosomal compartment. At the same time, the level of intracellular receptor was maintained at a constant level, suggesting that CTLA-4/AP-1 binding represents one mechanism to ensure steady state levels of intracellular CTLA-4 in T cells. Finally, we demonstrate that the TCR zeta/CD3 complex (but not CD28) also binds to AP-1 and AP-2 complexes, thus providing a possible link between these two receptors in the regulation of T cell function.

An intralysosomal hsp70 is required for a selective pathway of lysosomal protein degradation.

Previous studies have implicated the heat shock cognate (hsc) protein of 73 kD (hsc73) in stimulating a lysosomal pathway of proteolysis that is selective for particular cytosolic proteins. This pathway is activated by serum deprivation in confluent cultured human fibroblasts. We now show, using indirect immunofluorescence and laser scanning confocal microscopy, that a heat shock protein (hsp) of the 70-kD family (hsp70) is associated with lysosomes (ly-hsc73). An mAb designated 13D3 specifically recognizes hsc73, and this antibody colocalizes with an antibody to lgp120, a lysosomal marker protein. Most, but not all, lysosomes contain ly-hsc73, and the morphological appearance of these organelles dramatically changes in response to serum withdrawal; the punctate lysosomes fuse to form tubules. Based on susceptibility to digestion by trypsin and by immunoblot analysis after two-dimensional electrophoresis of isolated lysosomes and isolated lysosomal membranes, most ly-hsc73 is within the lysosomal lumen. We determined the functional importance of the ly-hsc73 by radiolabeling cellular proteins with [3H]leucine and then allowing cells to endocytose excess mAb 13D3 before measuring protein degradation in the presence and absence of serum. The increased protein degradation in response to serum deprivation was completely inhibited by endocytosed mAb 13D3, while protein degradation in cells maintained in the presence of serum was unaffected. The intralysosomal digestion of endocytosed [3H]RNase A was not affected by the endocytosed mAb 13D3. These results suggest that ly-hsc73 is required for a step in the degradative pathway before protein digestion within lysosomes, most likely for the import of substrate proteins.