ABSTRACT
Ras GTPase-activating protein-binding proteins 1 and 2 (G3BP1 and G3BP2, respectively) are widely recognized as core components of stress granules (SGs). We report that G3BPs reside at the cytoplasmic surface of lysosomes. They act in a non-redundant manner to anchor the tuberous sclerosis complex (TSC) protein complex to lysosomes and suppress activation of the metabolic master regulator mechanistic target of rapamycin complex 1 (mTORC1) by amino acids and insulin. Like the TSC complex, G3BP1 deficiency elicits phenotypes related to mTORC1 hyperactivity. In the context of tumors, low G3BP1 levels enhance mTORC1-driven breast cancer cell motility and correlate with adverse outcomes in patients. Furthermore, G3bp1 inhibition in zebrafish disturbs neuronal development and function, leading to white matter heterotopia and neuronal hyperactivity. Thus, G3BPs are not only core components of SGs but also a key element of lysosomal TSC-mTORC1 signaling.
Subject(s)
Adaptor Proteins, Signal Transducing/metabolism , DNA Helicases/metabolism , Lysosomes/metabolism , Mechanistic Target of Rapamycin Complex 1/metabolism , Poly-ADP-Ribose Binding Proteins/metabolism , RNA Helicases/metabolism , RNA Recognition Motif Proteins/metabolism , RNA-Binding Proteins/metabolism , Signal Transduction , Tuberous Sclerosis/metabolism , Amino Acid Sequence , Animals , Breast Neoplasms/metabolism , Breast Neoplasms/pathology , Cell Line, Tumor , Cell Movement/drug effects , Cytoplasmic Granules/drug effects , Cytoplasmic Granules/metabolism , DNA Helicases/chemistry , Evolution, Molecular , Female , Humans , Insulin/pharmacology , Lysosomal Membrane Proteins/metabolism , Lysosomes/drug effects , Neurons/drug effects , Neurons/metabolism , Phenotype , Poly-ADP-Ribose Binding Proteins/chemistry , RNA Helicases/chemistry , RNA Recognition Motif Proteins/chemistry , Rats, Wistar , Signal Transduction/drug effects , Zebrafish/metabolismABSTRACT
Advanced prefractionation strategies, in combination with highly sensitive and accurate mass spectrometers provide powerful means to detect and analyze low abundant proteins on the subcellular and organelle-specific level. Among enrichment techniques, subcellular fractionation has become the most commonly used. Its application gives access to less complex subproteomes and organelle constituents, facilitating downstream analysis. Furthermore, subcellular fractionation allows the identification of proteins that shuttle between different subcellular compartments in a stimulus dependent manner. As a paradigm of subcellular organelle isolation, we describe here endosomal purification protocols, based on differential centrifugation in continuous and discontinuous sucrose gradients. Described methods can be easily modified to isolate other organelles and are compatible with subsequent organelle- and functional organelle proteome analyses by, e.g., two-dimensional gel electrophoresis.
Subject(s)
Centrifugation, Density Gradient/methods , Endocytosis , Animals , Cell Line , Endosomes/chemistry , Humans , Proteome/analysis , Proteomics/methods , Sucrose/chemistryABSTRACT
The successful combination of highly sensitive mass spectrometry and pre-fractionation techniques has provided a powerful tool to detect dynamic changes in low abundant regulatory proteins at the organelle level. Subcellular fractionation, being flexible, adjustable (both in cell and tissues), and allowing the analysis of proteins in their physiologic/intracellular context, has become the most commonly used preparative/enrichment method. This chapter introduces state-of-the-art subcellular fractionation protocols and briefly discuss their suitability, advantages, and limitations.
