DNAJB12 and Hsp70 Mediate Triage of Misfolded Membrane Proteins for Proteasomal versus Lysosomal Degradation

Abstract

The endoplasmic reticulum (ER) fills the cell with a continuous network of sealed membrane tubules and sheets. The ER is subdivided into microdomains mediating one-third of total protein biosynthesis, oxidative protein folding, secretion, protein quality control, calcium signaling, marcoautophagy/autophagy, stress sensing, and apoptosis. Defects in ER-calcium homeostasis underlie several diseases. Damage to the ER by misfolded membrane proteins is suppressed by specific HSPA/Hsp70 and DNAJ/Hsp40 chaperone pairs that select intermediates for ubiquitination and ER-associated degradation (ERAD) via the proteasome. The ER-transmembrane Hsp40 chaperone DNAJB12 and HSPA/Hsp70 also target toxic intermediates of misfolded membrane proteins for ER-associated autophagy (ERAA). DNAJB12-HSPA/Hsp70 maintain membrane protein degradation intermediates in detergent-soluble and degradation-competent states. DNAJB12-HSPA/Hsp70 also interact with the autophagy initiation kinase ULK1 on ER tubules containing ERAD-resistant misfolded membrane proteins (ERAD-RMPs). Omegasomes are ER microdomains where the autophagosome precursor or phagophore (PG) forms. ER tubules loaded with ERAD-RMPs enter omegasomes where they are converted into ER-connected PG (ER-PG). The Atg8 (autophagy related 8)-family member GABARAP (GABA type A receptor-associated protein) facilitates transfer of ERAD-RMPs from ER-PGs to autolysosomes (AL) that dock transiently with omegasomes. This article describes a model for DNAJB12-HSPA/Hsp70 action during the conformation-dependent triage in the ER of misfolded membrane proteins for folding versus proteasomal or AL degradation. Abbreviations: ABC-transporter, ATP binding cassette transporter; Atg8, autophagy related 8; DNAJB12, DnaJ heat shock protein family (Hsp40) member B12; ER, endoplasmic reticulum; ERAD, ER-associated proteasomal degradation; ERAA, ER-associated autophagy; ERAD-RMPs, ER-associated proteasomal degradation resistant membrane protein; GABARAP, GABA type A receptor-associated protein; GPCR, G protein-coupled receptor; HSPA/Hsp70, heat shock protein family A (Hsp70); LAMP1, lysosomal associated membrane protein 1; MAP1LC3/LC3, microtubule associated protein 1 light chain 3; P-type ATPases, ion transporting ATPase; ULK1, unc-51 like autophagy activating kinase 1; RB1CC1/FIP200, RB1 inducible coiled-coil 1; WIPI, WD repeat domain, phosphoinositide interacting.