Pervasive, Coordinated Protein-Level Changes Driven by Transcript Isoform Switching during Meiosis.

To better understand the gene regulatory mechanisms that program developmental processes, we carried out simultaneous genome-wide measurements of mRNA, translation, and protein through meiotic differentiation in budding yeast. Surprisingly, we observed that the levels of several hundred mRNAs are anti-correlated with their corresponding protein products. We show that rather than arising from canonical forms of gene regulatory control, the regulation of at least 380 such cases, or over 8% of all measured genes, involves temporally regulated switching between production of a canonical, translatable transcript and a 5' extended isoform that is not efficiently translated into protein. By this pervasive mechanism for the modulation of protein levels through a natural developmental program, a single transcription factor can coordinately activate and repress protein synthesis for distinct sets of genes. The distinction is not based on whether or not an mRNA is induced but rather on the type of transcript produced.

Seq-ing answers: uncovering the unexpected in global gene regulation.

The development of techniques for measuring gene expression globally has greatly expanded our understanding of gene regulatory mechanisms in depth and scale. We can now quantify every intermediate and transition in the canonical pathway of gene expression-from DNA to mRNA to protein-genome-wide. Employing such measurements in parallel can produce rich datasets, but extracting the most information requires careful experimental design and analysis. Here, we argue for the value of genome-wide studies that measure multiple outputs of gene expression over many timepoints during the course of a natural developmental process. We discuss our findings from a highly parallel gene expression dataset of meiotic differentiation, and those of others, to illustrate how leveraging these features can provide new and surprising insight into fundamental mechanisms of gene regulation.

Developmentally regulated selective autophagy determines ER inheritance by gametes.

The endoplasmic reticulum (ER) carries out essential cellular functions ranging from protein trafficking to metabolite signaling. ER function is maintained in part by quality control pathways including ER degradation by selective autophagy (reticulophagy) during conditions of cellular stress. Reticulophagy is known to be important for cellular responses to starvation and protein folding stress, but no natural role during development had been identified. While investigating ER remodeling during the conserved cell differentiation process of meiosis in budding yeast, we unexpectedly observed developmentally regulated reticulophagy that was driven by expression of the autophagy receptor Atg40. This reticulophagy was coordinated with massive morphological rearrangement of the ER, including movement of most cortical ER away from the cell periphery. As meiotic reticulophagy prevents specific ER subpopulations from being inherited by gametes, we propose that it serves a quality control role, preventing deleterious material from being passed on to subsequent generations.

Programmed cortical ER collapse drives selective ER degradation and inheritance in yeast meiosis.

The endoplasmic reticulum (ER) carries out essential and conserved cellular functions, which depend on the maintenance of its structure and subcellular distribution. Here, we report developmentally regulated changes in ER morphology and composition during budding yeast meiosis, a conserved differentiation program that gives rise to gametes. A subset of the cortical ER collapses away from the plasma membrane at anaphase II, thus separating into a spatially distinct compartment. This programmed collapse depends on the transcription factor Ndt80, conserved ER membrane structuring proteins Lnp1 and reticulons, and the actin cytoskeleton. A subset of ER is retained at the mother cell plasma membrane and excluded from gamete cells via the action of ER-plasma membrane tethering proteins. ER remodeling is coupled to ER degradation by selective autophagy, which relies on ER collapse and is regulated by timed expression of the autophagy receptor Atg40. Thus, developmentally programmed changes in ER morphology determine the selective degradation or inheritance of ER subdomains by gametes.

Global Proteome Remodeling during ER Stress Involves Hac1-Driven Expression of Long Undecoded Transcript Isoforms.

Cellular stress responses often require transcription-based activation of gene expression to promote cellular adaptation. Whether general mechanisms exist for stress-responsive gene downregulation is less clear. A recently defined mechanism enables both up- and downregulation of protein levels for distinct gene sets by the same transcription factor via coordinated induction of canonical mRNAs and long undecoded transcript isoforms (LUTIs). We analyzed parallel gene expression datasets to determine whether this mechanism contributes to the conserved Hac1-driven branch of the unfolded protein response (UPRER), indeed observing Hac1-dependent protein downregulation accompanying the upregulation of ER-related proteins that typifies UPRER activation. Proteins downregulated by Hac1-driven LUTIs include those with electron transport chain (ETC) function. Abrogated ETC function improves the fitness of UPRER-activated cells, suggesting functional importance to this regulation. We conclude that the UPRER drives large-scale proteome remodeling, including coordinated up- and downregulation of distinct protein classes, which is partly mediated by Hac1-induced LUTIs.