Polyansky, A. | Search bioRxiv

bioRxiv2023

Rim aperture of autophagic membranesbalances cargo inclusion with vesicle maturation

Autophagy eliminates cytoplasmic material by engulfment in membranous vesicles targeted for lysosome degradation. Nonselective autophagy coordinates the sequestration of bulk cargo with the growth of the isolation membrane (IM) in a yet-unknown manner. Here we show the yeast IM expands while maintaining a rim sufficiently wide for sequestration of large cargo but tight enough to mature in due time. An obligate complex of Atg24/Snx4 with Atg20 or Snx41 assembles locally at the rim in a spatially-extended manner that specifically depends on autophagic PI(3)P. This assembly stabilizes the open rim to promote autophagic sequestration of large cargo in correlation with vesicle inflation. Moreover, constriction of the rim by the PI(3)P-dependent Atg2-Atg18 complex and clearance of PI(3)P by Ymr1 antagonize the rim opening to promote autophagic maturation and consumption of small cargo. Tight regulation of membrane rim aperture by PI(3)P thus couples the mechanism and physiology of nonselective autophagy.

cell biology↗

bioRxiv2022

The SPOC domain is a phosphoserine binding module that bridges transcription machinery with co- and post-transcriptional regulators

The heptarepeats of the C-terminal domain (CTD) of RNA polymerase II (Pol II) are extensively modified throughout the transcription cycle. The CTD coordinates RNA synthesis and processing by recruiting transcription regulation factors as well as RNA capping, splicing and 3end processing factors. The SPOC domain of PHF3 was recently identified as a new CTD reader domain specifically binding to phosphorylated serine-2 residues in adjacent CTD repeats. Here, we establish the SPOC domains of the human proteins DIDO, SHARP and RBM15 as phosphoserine binding modules that can act as CTD readers but also recognize other phosphorylated binding partners. We report the crystal structure of SHARP (SPEN) SPOC-CTD and identify the molecular determinants for its specific binding to phosphorylated serine-5. PHF3 and DIDO SPOC domains preferentially interact with the Pol II elongation complex, while RBM15 and SHARP SPOC domains engage with the m6A writer and reader proteins. Our findings establish the SPOC domain as a major interface between the transcription machinery and regulators of transcription and co-transcriptional processes.

biochemistry↗