SETD2 transcriptional control of ATG14L/S isoforms regulates autophagosome-lysosome fusion

Macroautophagy/autophagy is an evolutionarily conserved and tightly regulated catabolic process involved in the maintenance of cellular homeostasis whose dysregulation is implicated in several pathological processes. Autophagy begins with the formation of phagophores that engulf cytoplasmic cargo and mature into double-membrane autophagosomes; the latter fuse with lysosomes/vacuoles for cargo degradation and recycling. Here, we report that yeast Set2, a histone lysine methyltransferase, and its mammalian homolog, SETD2, both act as positive transcriptional regulators of autophagy. However, whereas Set2 regulates the expression of several autophagy-related (Atg) genes upon nitrogen starvation, SETD2 effects in mammals were found to be more restricted. In fact, SETD2 appears to primarily regulate the differential expression of protein isoforms encoded by the ATG14 gene. SETD2 promotes the expression of a long ATG14 isoform, ATG14L, that contains an N-terminal cysteine repeats domain, essential for the efficient fusion of the autophagosome with the lysosome, that is absent in the short ATG14 isoform, ATG14S. Accordingly, SETD2 loss of function decreases autophagic flux, as well as the turnover of aggregation-prone proteins such as mutant HTT (huntingtin) leading to increased cellular toxicity. Hence, our findings bring evidence to the emerging concept that the production of autophagy-related protein isoforms can differentially affect core autophagy machinery bringing an additional level of complexity to the regulation of this biological process in more complex organisms.