Lipid droplets (LDs) are the major lipid storage organelles of eukaryotic cells and a source of nutrients for intracellular pathogens. We demonstrate that mammalian LDs are endowed with a protein-mediated antimicrobial capacity, which is up-regulated by danger signals. In response to lipopolysaccharide (LPS), multiple host defense proteins, including interferon-inducible guanosine triphosphatases and the antimicrobial cathelicidin, assemble into complex clusters on LDs. LPS additionally promotes the physical and functional uncoupling of LDs from mitochondria, reducing fatty acid metabolism while increasing LD-bacterial contacts. Thus, LDs actively participate in mammalian innate immunity at two levels: They are both cell-autonomous organelles that organize and use immune proteins to kill intracellular pathogens as well as central players in the local and systemic metabolic adaptation to infection.

The mitochondria-associated membrane (MAM) is a specialized subdomain of the endoplasmic reticulum (ER) which acts as an intracellular signaling hub. MAM dysfunction has been related to liver disease. We report a high-throughput mass spectrometry-based proteomics characterization of MAMs from mouse liver, which portrays them as an extremely complex compartment involved in different metabolic processes, including steroid metabolism. Interestingly, we identified caveolin-1 (CAV1) as an integral component of hepatic MAMs, which determine the relative cholesterol content of these ER subdomains. Finally, a detailed comparative proteomics analysis between MAMs from wild type and CAV1-deficient mice suggests that functional CAV1 contributes to the recruitment and regulation of intracellular steroid and lipoprotein metabolism-related processes accrued at MAMs. The potential impact of these novel aspects of CAV1 biology on global cell homeostasis and disease is discussed.

SummaryThe transcriptional regulator YAP orchestrates many cellular functions, including tissue homeostasis, organ growth control, and tumorigenesis. Mechanical stimuli are a key input to YAP activity, but the mechanisms controlling this regulation remain largely uncharacterized. We show that CAV1 positively modulates the YAP mechanoresponse to substrate stiffness through actin-cytoskeleton-dependent and Hippo-kinase-independent mechanisms. RHO activity is necessary, but not sufficient, for CAV1-dependent mechanoregulation of YAP activity. Systematic quantitative interactomic studies and image-based small interfering RNA (siRNA) screens provide evidence that this actin-dependent regulation is determined by YAP interaction with the 14-3-3 protein YWHAH. Constitutive YAP activation rescued phenotypes associated with CAV1 loss, including defective extracellular matrix (ECM) remodeling. CAV1-mediated control of YAP activity was validated in vivo in a model of pancreatitis-driven acinar-to-ductal metaplasia. We propose that this CAV1-YAP mechanotransduction system controls a significant share of cell programs linked to these two pivotal regulators, with potentially broad physiological and pathological implications.

, respectively. WW and FF domains may, therefore, serve to link transcription and splicing components and play a role in coupling transcription and splicing in vivo. In the study presented here, we investigated the subcellular localization and association of CA150 with factors involved in pre-mRNA transcriptional elongation and splicing. Endogenous CA150 colocalized with nuclear speckles, and this was not affected either by inhibition of cellular transcription or by RNAPII CTD phosphorylation. FF domains are essential for the colocalization to speckles, while WW domains are not required for colocalization. We also performed biochemical assays to understand the role of WW and FF domains in mediating the assembly of transcription and splicing components into higher-order complexes. Transcription and splicing components bound to a region in the amino-terminal part of CA150 that contains the three WW domains; however, we identified a region of the C-terminal FF domains that was also critical. Our results suggest that sequences located at both the amino and carboxyl regions of CA150 are required to assemble transcription/splicing complexes, which may be involved in the coupling of those processes.Expression of protein-encoding genes is a multistep process beginning with transcription by RNA polymerase II (RNAPII). During transcription, the nascent pre-mRNA undergoes several processing steps, including capping, splicing, and polyadenylation. Distinct cellular machines carry out each of the steps in gene expression, but growing evidence indicates that there are coupled interactions between these machineries and it is now believed that most mRNA processing reactions occur cotranscriptionally (23, 51, 54). The C-terminal repeat domain (CTD) of RNAPII has a principal role in coupling eukaryotic transcription and pre-mRNA processing. The CTD of RNAPII is composed of 52 (in mammals) or 26 (in yeast [Saccharomyces cerevisiae]) tandem repeats of the consensus heptapeptide YS PTSPS, and this domain is essential for viability in yeast (11). An RNAPII with an unphosphorylated CTD assembles into preinitiation complexes at the promoters, whereas the transition between initiation and elongation is accompanied by multiple phosphorylation events of the CTD. These events are catalyzed by several protein kinases, including cyclin-dependent kinase 7 (CDK7), which is part of the general transcription factor IIH (TFIIH) (14, 35), and CDK9, which is the kinase component of positive transcription elongation factor b (P-TEFb) (37,38). Phosphorylation of the CTD on Ser5 correlates with the presence of RNAPII at the promoter, while phosphorylation on Ser2 correlates with RNAPII at the coding regions of genes and thus is considered a mark of elongating polymerases (30). As first proposed by Greenleaf (19), the CTD of RNAPII has been shown to interact with capping, splicing, and polyadenylation factors (8,9,25,39,40), thus acting as a platform upon which mRNA processing factors bind. It is most likely that the phosphorylation events occurring at...