m unique spectrum counts for autophagy receptors including p62 f from mass spectrometry of ythdf2 - interacting proteins as compared with igg. n immunoblot of ythdf2, p62, and lc3 - i / ii in hacat cells expressing shnc or shp62, 6 h after sham or uvb irradiation. o co - ip of flag - tagged ythdf2 with ha - tagged p62 ( wt or δuba ) in hela cells. p co - ip of flag - tagged ythdf2 ( wt, n - terminus or c - terminus ) with p62 in hela cells. housekeeping gene: 18s rrna ( e, g, h ). statistical analyses were performed using a two - tailed unpaired student ’ s t - test. data are shown as mean ± sd ( n = 3 for e – j ). all experiments were conducted using biologically independent samples. a loss of ythdf2 phosphorylation at serine 39 ( s39 ) in hacat cells after uvb irradiation identified by mass spectrometry. b immunoblot of p - ythdf2 ( s39 ) and ythdf2 in hacat cells with or without uvb irradiation ( 1 h ). c co - ip of mypt1 with flag - tagged ythdf2 ( ev, wt, s39d or s39a ) in df2 ko hela cells. d co - ip of cnot1 with flag - tagged ythdf2 ( ev, wt, s39d or s39a ) in df2 ko hela cells. e qpcr of u6 snrna stability in df2 ko hela cells transfected with df2 wt or df2 s39a. f rip of u6 with cnot1 in hela cells. qpcr of u6 snrna stability in hela ( g ) and a431 ( h ) cells with or without cnot1 knockdown. rip of u6 with cnot1 in hela cells with or without ythdf2 knockout ( i ) or mettl16 knockdown ( j ). k immunoblot of flag - tagged ythdf2 in wcl and endosomes from hela cells transfected with ev, df2 wt or df2 s39a. l immunoblot analysis of ythdf2, p62, and lc3 - i / ii in hacat after sham ( c ) or uvb irradiation. m unique spectrum counts for autophagy receptors including p62 f from mass spectrometry of ythdf2 - interacting proteins as compared with igg. n immunoblot of ythdf2, p62, and lc3 - i / ii in hacat cells expressing shnc or shp62, 6 h after sham or uvb irradiation. o co - ip of flag - tagged ythdf2 with ha - tagged p62 ( wt or δuba ) in hela cells. p co - ip of flag - tagged ythdf2 ( wt, n - terminus or c - terminus ) with p62 in hela cells. housekeeping gene: 18s rrna ( e, g, h ). statistical analyses were performed using a two - tailed unpaired student ’ s t - test. data are shown as mean ± sd ( n = 3 for e – j ). all experiments were conducted using biologically independent samples. next, we assessed the role of cnot1 in u6 decay. indeed, we found that cnot1 binds to u6 in hela cells ( fig. 6f ) and cnot1 knockdown inhibits u6 decay in both hela and a431 cells ( fig. 6g, h, and supplementary fig. s15j, k ). to elucidate the mechanism of ythdf2 - mediated u6 decay, we examined the role of ythdf2 and mettl16 - mediated ma methylation in cnot1 binding to u6 snrna. we found that either ythdf2 deletion or mettl16 knockdown drastically reduces cnot1 binding to u6 in hela cells ( fig. 6i, j and supplementary fig. s15l, m ), supporting a critical role of ythdf2 and u6 ma methylation in cnot1 binding to u6 snrna. to determine whether uvb irradiation affects cnot1 expression, we assessed the effect of uvb irradiation on cnot1 protein levels. we found that uvb irradiation had no effect on cnot1 protein expression in either hela or hacat cells ( supplementary fig. s15n ). these results demonstrate that cnot1 binds to u6 snrna and mediates u6 decay through interacting with ythdf2 in an ma - methylation dependent manner. furthermore, the s39a mutant reduced endosomal localization in hela cells ( fig. 6k ). these findings demonstrate that cnot1 mediates u6 decay and dephosphorylation of ythdf2 inhibits u6 decay and endosomal localization. in addition to inhibiting ythdf2 phosphorylation, we observed that uvb down-regulate s ythdf2 protein in multiple cell lines and primary keratinocytes including hacat, chl - 1 ( melanoma cells ), hela, and nhek cells ( fig. 6l, and supplementary fig. s16a – c ). this occurred in parallel with autophagy induction, as shown by the downregulation of the selective autophagy receptor p62 and the induction of lc3 - ii, despite the increase in the ythdf2 mrna level in hacat cells ( fig. 6l, and supplementary fig. s16a - d ). uvb irradiation induces an acute inflammatory response as observed by increased expression of inflammatory genes ( fig. 1 ). to determine whether the inflammatory response correlates with ythdf2 protein abundance, we assessed ythdf2 protein levels at different time points. indeed, as compared with sham - irradiation, ythdf2 protein level is decreased at 6 h post - uvb irradiation, while it was recovered at 24 h post - uvb irradiation in hacat cells ( supplementary fig. s16d ), likely due to increased ythdf2 mrna levels ( supplementary fig. s16d ). the temporal regulation of ythdf2 protein expression by uvb irradiation negatively correlated with the increases in u6 snrna levels ( fig. 2i ) as well as inflammatory gene expression at 6 h followed by a decrease at 24 h post - uvb irradiation, further supporting an inhibitory role of ythdf2 in uvb - induced inflammation. importantly, uvb irradiation had little effect on other ma readers in hacat cells ( supplementary fig. s16e ). furthermore, uvb irradiation also down-regulate d ythdf2 protein in mouse skin ( supplementary fig. s16f, g ). next, to determine whether ythdf2 down - regulation induced by uvb radiation is mediated through autophagy, a cellular self - eating process that degrades excessive or damaged proteins and organelles, we sought to establish whether autophagy inhibition reversed the effect of uvb irradiation on ythdf2 and u6 levels. using hacat cells with or without knockdown of the essential autophagy gene atg5 or atg7, we found that inhibition of either atg5 or atg7 prevented uvb - induced down - regulation of ythdf2 and u6 up - regulation ( supplementary fig. s16h – j ). to identify the autophagy receptor responsible for uvb - induced autophagy of ythdf2, we re - examined the list of ythdf2 - interacting proteins from our mass spectrometry analysis. we observed that ythdf2 preferentially binds with the autophagy receptor p62 ( also known as sqstm1 ) in hacat cells ( fig. 6m, and supplementary fig. s16k ). indeed, knockdown of p62 prevented uvb - induced ythdf2 down - regulation and u6 up - regulation in hacat cells ( fig. 6n and supplementary fig. 16l ). furthermore, in mouse skin, skin - specific conditional knockout ( cko ) of atg5, atg7, or sqstm1 ( p62 ) prevented uvb - induced ythdf2 down - regulation ( supplementary fig. s16m, n ). co - ip showed that p62 binds to ythdf2 in hela and hacat cells. additionally, the p62 - ythdf2 interaction requires p62 ’ s ubiquitin - associated ( uba ) domain in hela cells ( fig. 6o and supplementary fig. s16o ), which is known to interact with ubiquitinated proteins. as both p62 and ythdf2 can bind to rna, we assessed whether the p62 - ythdf2 interaction is dependent on rna. we found that removal of rna had no effect on the interaction between p62 and ythdf2 in hacat cells ( supplementary fig. s16p ). in addition, we found that the c - terminus, but not the n - terminus, of ythdf2 is required for ythdf2 binding to p62 ( fig. 6p ). these results demonstrate that ythdf2 is degraded via p62 - dependent selective autophagy in response to uvb stress. as inflammation can promote tumorigenesis and the role of ythdf2 in established cancers is cancer - type - dependent, we next investigated the function of ythdf2 in skin tumorigenesis. indeed, our rna - seq data analysis showed that genes in the pathways in cancer are upregulated by ythdf2 knockdown in hacat cells ( fig. 1a ), suggesting a role for ythdf2 in skin cancer. in addition, we found that knockdown of ythdf2 in a431 cells increases cell proliferation and migration in vitro, whereas forced overexpression of ythdf2 in a431 cells decreases cell proliferation and migration ( fig. 7a – c, and supplementary fig. s17a ).