Hsc70-4 Deforms Membranes to Promote Synaptic Protein Turnover by Endosomal Microautophagy

Synapses are often far from their cell bodies and must largely independently cope with dysfunctional proteins resulting from synaptic activity and stress. To identify membrane-associated machines that can engulf synaptic targets destined for degradation, we performed a large-scale in vitro liposome-based screen followed by functional studies. We identified a presynaptically enriched chaperone Hsc70-4 that bends membranes based on its ability to oligomerize. This activity promotes endosomal microautophagy and the turnover of specific synaptic proteins. Loss of microautophagy slows down neurotransmission while gain of microautophagy increases neurotransmission. Interestingly, Sgt, a cochaperone of Hsc70-4, is able to switch the activity of Hsc70-4 from synaptic endosomal microautophagy toward chaperone activity. Hence, Hsc70-4 controls rejuvenation of the synaptic protein pool in a dual way: either by refolding proteins together with Sgt, or by targeting them for degradation by facilitating endosomal microautophagy based on its membrane deforming activity. •The Hsc70-4 chaperone can deform membranes•Hsc70-4-dependent membrane deformation is required for synaptic microautophagy•Hsc70-4 promotes turnover of numerous synaptic proteins by endosomal microautophagy•This quality control pathway regulates the efficiency of neurotransmitter release Uytterhoeven, Lauwers, and Maes et al. conducted a proteome-wide screen for membrane-deforming proteins and found the chaperone Hsc70-4. They show that Hsc70-4-dependent membrane deformation promotes endosomal microautophagy at presynaptic terminals, facilitating the degradation of synaptic proteins and promoting neurotransmitter release.