Structure of Transmembrane Domain of Lysosomal-Associated Membrane Protein Type 2a (LAMP-2A) Reveals Key Features for Substrate Specificity in Chaperone Mediated Autophagy [Molecular Biophysics]

October 22nd, 2014 by Rout, A. K., Strub, M.-P., Piszczek, G., Tjandra, N.

Chaperone-mediated autophagy (CMA) is a highly regulated cellular process that mediates the degradation of a selective subset of cytosolic proteins in lysosomes. Increasing CMA activity is one way for a cell to respond to stress, which leads to enhanced turnover of non-critical cytosolic proteins into sources of energy or clearance of unwanted or damaged proteins from the cytosol. The lysosome-associated membrane protein type 2a (LAMP-2A) together with a complex of chaperones and co-chaperones are key regulators of CMA. LAMP-2A is a trans-membrane protein component for protein translocation to the lysosome. Here we present a study of the structure and dynamics of the transmembrane domain (TM) of human LAMP-2A in n-dodecylphosphocholine (DPC) micelles by nuclear magnetic resonance (NMR). We showed that LAMP-2A exists as a homo-trimer in which the membrane spanning helices wrap around each other to form a parallel coiled-coil conformation, whereas its cytosolic tail is flexible and exposed to the cytosol. This cytosolic tail of LAMP-2A interacts with chaperone Hsc70 and a CMA substrate RNase A with comparable affinity, but not with Hsp40 and RNase S peptide. Since the substrates and the chaperone complex can bind at the same time thus creating a bimodal interaction, we propose that substrate recognition by chaperones and targeting to the lysosomal membrane by Lamp-2A are coupled. This can increase substrate affinity and specificity as well as avoiding substrate aggregation and assisting in the unfolding of the substrate as well as promoting the formation of higher order complex of LAMP-2A required for translocation.
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