Cytokines are protein mediators that are known to be involved in many biological processes, including cell growth, survival, inflammation, and development. To study their regulation, we generated a library of 209 different cytokines. This was used in a combinatorial format to study the effects of cytokines on each other, with particular reference to the control of differentiation. This study showed that IFN-γ is a master checkpoint regulator for many cytokines. It operates via an autocrine mechanism to elevate STAT1 and induce internalization of gp130, a common component of many heterodimeric cytokine receptors. This targeting of a receptor subunit that is common to all members of an otherwise diverse family solves the problem of how a master regulator can control so many diverse receptors. When one adds an autocrine mechanism, fine control at the level of individual cells is achieved.A s our understanding of physiology grows, we are increasingly aware of its Newtonian aspects in that for every action there is another reaction with opposite consequences (1). Irrespective of whether one thinks in terms of homeostasis, feedback loops, or checkpoints, we increasingly uncover molecular systems that, in terms of function, induce cells to move in opposite ways. For example, although immune T-cell activation is critical in controlling disease, one needs a checkpoint to guard against overactivity that could result in autoimmunity (2, 3).In addition to adding to our general knowledge of cellular physiology, detailed understanding of the molecular mechanisms of activation and checkpoint processes has important therapeutic implications. Such an understanding allows two separate entry points into the regulation of cellular events. Thus, if one wants to promote a cellular function, the same outcome can be achieved by either enhancing the effector or inhibiting the checkpoint pathways, usually by perturbing the molecules that initiate them.One of the most important physiological systems is the cytokine cascade (4). However, here, because the cascade contains large numbers of separate molecules, each operating through different receptors, simple models of regulation break down. While one can imagine that each cytokine is paired with a separate checkpoint system operating in an opposite direction, it seemed to us to be unlikely, if for no other reason than the economical use of genetic information. Given that one had large sets of related molecules with overlapping signal transduction mechanisms and pathways, it seemed more reasonable to assume that, depending on the circumstance, some members of the cytokine repertoire regulate other members of the repertoire. This differs from situations such as PD-1, where the checkpoint mechanistic cascade differs completely from the activation mechanism. By contrast, we propose that cytokine members of a family regulate each other by perturbing common molecular mechanisms. Thus, if gene action economy is proposed for a family, its hallmark should be that some of the same molecules used for...

Highlights d Dfm1 selectively binds ERAD-targeted membrane substrates d Polyubiquitin chains bind directly to Cdc48 recruited by Dfm1 d Derlin lipid thinning facilitates removal of integral membrane substrates in the ER d Substrate engagement and lipid thinning are conserved derlin features

Summary ER-associated degradation (ERAD) targets misfolded ER proteins for degradation. Retrotranslocation, a key feature of ERAD, entails removal of ubiquitinated substrates into the cytosol for proteasomal destruction. Recently, it has been shown that the Hrd1 E3 ligase forms a retrotranslocation channel for luminal (ERAD-L) substrates. Conversely, our studies found that integral membrane (ERAD-M) substrates exit the ER through a distinct pathway mediated by the Dfm1 rhomboid protein. Those studies also revealed a second, Hrd1-dependent pathway of ERAD-M retrotranslocation can arise in dfm1Δ null. Here we show that, in the dfm1Δ null, the HRD complex undergoes remodeling to a form that mediates ERAD-M retrotranslocation. Specifically, Hrd1's normally present stochiometric partner Hrd3 is efficiently removed during suppressive remodeling, allowing Hrd1 to function in this novel capacity. Neither Hrd1 autoubiquitination nor its cytosolic domain is required for suppressive ERAD-M retrotranslocation. Thus, the HRD complex displays remarkable functional flexibility in response to ER stress.

Phosphatidylethanolamine (PE) made in mitochondria has long been recognized as an important precursor for phosphatidylcholine production that occurs in the endoplasmic reticulum (ER). Recently, the strict mitochondrial localization of the enzyme that makes PE in the mitochondrion, phosphatidylserine decarboxylase 1 (Psd1), was questioned. Since a dual localization of Psd1 to the ER would have far-reaching implications, we initiated our study to independently re-assess the subcellular distribution of Psd1. Our results support the unavoidable conclusion that the vast majority, if not all, of functional Psd1 resides in the mitochondrion. Through our efforts, we discovered that mutant forms of Psd1 that impair a self-processing step needed for it to become functional are dually localized to the ER when expressed in a PE-limiting environment. We conclude that severely impaired cellular PE metabolism provokes an ER-assisted adaptive response that is capable of identifying and resolving nonfunctional mitochondrial precursors.

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