Huntingtin-interacting protein 1 (Hip1) and Hip1-related protein (Hip1R) bind the conserved sequence of clathrin light chains and thereby influence clathrin assembly in vitro and actin distribution in vivo.
Clathrin heavy and light chains form triskelia, which assemble into polyhedral coats of membrane vesicles that mediate transport for endocytosis and organelle biogenesis. Light chain subunits regulate clathrin assembly in vitro by suppressing spontaneous self-assembly of the heavy chains. The residues that play this regulatory role are at the N terminus of a conserved 22-amino acid sequence that is shared by all vertebrate light chains. Here we show that these regulatory residues and others in the conserved sequence mediate light chain interaction with Hip1 and Hip1R. These related proteins were previously found to be enriched in clathrin-coated vesicles and to promote clathrin assembly in vitro. We demonstrate Hip1R binding preference for light chains associated with clathrin heavy chain and show that Hip1R stimulation of clathrin assembly in vitro is blocked by mutations in the conserved sequence of light chains that abolish interaction with Hip1 and Hip1R. In vivo overexpression of a fragment of clathrin light chain comprising the Hip1R-binding region affected cellular actin distribution. Together these results suggest that the roles of Hip1 and Hip1R in affecting clathrin assembly and actin distribution are mediated by their interaction with the conserved sequence of clathrin light chains. (+info)
Huntingtin interacting protein 1 (HIP1) regulates clathrin assembly through direct binding to the regulatory region of the clathrin light chain.
Huntingtin interacting protein 1 (HIP1) is a component of clathrin coats. We previously demonstrated that HIP1 promotes clathrin assembly through its central helical domain, which binds directly to clathrin light chains (CLCs). To better understand the relationship between CLC binding and clathrin assembly we sought to dissect this interaction. Using C-terminal deletion constructs of the HIP1 helical domain, we identified a region between residues 450 and 456 that is required for CLC binding. Within this region, point mutations showed the importance of residues Leu-451, Leu-452, and Arg-453. Mutants that fail to bind CLC are unable to promote clathrin assembly in vitro but still mediate HIP1 homodimerization and heterodimerization with the family member HIP12/HIP1R. Moreover, HIP1 binding to CLC is necessary for HIP1 targeting to clathrin-coated pits and clathrin-coated vesicles. Interestingly, HIP1 binds to a highly conserved region of CLC previously demonstrated to regulate clathrin assembly. These results suggest a role for HIP1/CLC interactions in the regulation of clathrin assembly. (+info)
Clathrin heavy and light chain isoforms originated by independent mechanisms of gene duplication during chordate evolution.
In humans, there are two isoforms each of clathrin heavy chain (CHC17 and CHC22) and light chain (LCa and LCb) subunits, all encoded by separate genes. CHC17 forms the ubiquitous clathrin-coated vesicles that mediate membrane traffic. CHC22 is implicated in specialized membrane organization in skeletal muscle. CHC17 is bound and regulated by LCa and LCb, whereas CHC22 does not functionally interact with either light chain. The imbalanced interactions between clathrin subunit isoforms suggest a distinct evolutionary history for each isoform pair. Phylogenetic and sequence analysis placed both heavy and light chain gene duplications during chordate evolution, 510-600 million years ago. Genes encoding CHC22 orthologues were found in several vertebrate species, with only a pseudogene present in mice. Multiple paralogons surrounding the CHC genes (CLTC and CLTD) were identified, evidence that genomic or large-scale gene duplication produced the two CHC isoforms. In contrast, clathrin light chain genes (CLTA and CLTB) apparently arose by localized duplication, within 1-11 million years of CHC gene duplication. Analysis of sequence divergence patterns suggested that structural features of the CHCs were maintained after gene duplication, but new interactions with regulatory proteins evolved for the CHC22 isoform. Thus, independent mechanisms of gene duplication expanded clathrin functions, concomitant with development of neuromuscular sophistication in chordates. (+info)
Non-stoichiometric relationship between clathrin heavy and light chains revealed by quantitative comparative proteomics of clathrin-coated vesicles from brain and liver.
We used tandem mass spectrometry with peptide counts to identify and to determine the relative levels of expression of abundant protein components of highly enriched clathrin-coated vesicles (CCVs) from rat liver. The stoichiometry of stable protein complexes including clathrin heavy chain and clathrin light chain dimers and adaptor protein (AP) heterotetramers was assessed. We detected a deficit of clathrin light chain compared with clathrin heavy chain in non-brain tissues, suggesting a level of regulation of clathrin cage formation specific to brain. The high ratio of AP-1 to AP-2 in liver CCVs is reversed compared with brain where there is more AP-2 than AP-1. Despite this, general endocytic cargo proteins were readily detected in liver but not in brain CCVs, consistent with the previous demonstration that a major function for brain CCVs is recycling synaptic vesicles. Finally we identified 21 CCV-associated proteins in liver not yet characterized in mammals. Our results further validate the peptide accounting approach, reveal new information on the properties of CCVs, and allow for the use of quantitative proteomics to compare abundant components of organelles under different experimental and pathological conditions. (+info)
Membrane-associated STAT3 and PY-STAT3 in the cytoplasm.
Signal transduction from the plasma membrane to the nucleus by STAT proteins is widely represented as exclusively a soluble cytosolic process. Using cell-fractionation methods, we observed that approximately 5% of cytoplasmic STAT3 was constitutively associated with the purified early endosome (EE) fraction in human Hep3B liver cells. By 15-30 min after interleukin-6 (IL-6) treatment, up to two-thirds of cytoplasmic Tyr-phosphorylated STAT3 can be associated with the purified early endosome fraction (Rab-5-, EEA1-, transferrin receptor-, and clathrin-positive fraction). Electron microscopy, immunofluorescence, and detergent dissection approaches confirmed the association of STAT3 and PY-STAT3 with early endosomes. STAT3 was constitutively associated with clathrin heavy chain in membrane and in the 1- to 2-MDa cytosolic complexes. The membrane association was dynamic in that, within 15 min of treatment with the vicinal-thiol cross-linker phenylarsine oxide, there was a dramatic increase in bulk STAT3 association with sedimentable membranes. The functional contribution of PY-STAT3 association with the endocytic pathway was evaluated in transient transfection assays using IL-6-inducible STAT3-reporter-luciferase constructs and selective regulators of this pathway. STAT3-transcriptional activation was inhibited by expression constructs for dominant negative dynamin K44A, epsin 2a, amphiphysin A1, and clathrin light chain but enhanced by that for the active dynamin species MxA. Taken together, these studies emphasize the contribution of the endocytic pathway to productive IL-6/STAT3 signaling. (+info)
Clathrin light chain b is capable of affecting potently a major protein phosphatase from microtubules (MT-PP1).
Clathrin light chain (CL) b purified from bovine brain postmicrotubule supernatant and identified by mass spectrometry potently inhibited a catalytic activity of a major protein phosphatase (PP) that was copurified with microtubules and recognized by antiPP1 antibodies. CLb similarly affected the catalytic subunit and holoenzyme of the PP, little inhibiting the activity of PP2A. Although the CLb from clathrin-coated vesicles was several hundredfold weaker than our purified CLb, the CLb in the postmicrotubule supernatant, independent of whether it was sedimentable or soluble, was as active as the purified CLb. Thus CLb may be a potent regulator of the PP. (+info)
Calcyon, a novel partner of clathrin light chain, stimulates clathrin-mediated endocytosis.
In the central nervous system, clathrin-mediated endocytosis is crucial for efficient synaptic transmission. Clathrin-coated vesicle assembly and disassembly is regulated by some 30 adaptor and accessory proteins, most of which interact with clathrin heavy chain. Using the calcyon cytosolic domain as bait, we isolated clathrin light chain in a yeast two-hybrid screen. The interaction domain was mapped to the heavy chain binding domain and C-terminal regions of light chain. Further, the addition of the calcyon C terminus stimulated clathrin self-assembly in a dose-dependent fashion. Calcyon, which is a single transmembrane protein predominantly expressed in brain, localized to vesicular compartments within pre- and postsynaptic structures. There was a high degree of overlap in the distribution of LC and calcyon in neuronal dendrites, spines, and cell bodies. Co-immunoprecipitation studies further suggested an association of calcyon with the clathrin-mediated endocytic machinery. Compared with controls, HEK293 cells overexpressing calcyon exhibited significantly enhanced transferrin uptake but equivalent levels of recycling. Conversely, transferrin uptake was largely abolished in neocortical neurons obtained from mice homozygous for a calcyon null allele, whereas recycling proceeded at wild type levels. Collectively, these data indicate a role for calcyon in clathrin-mediated endocytosis in brain. (+info)
Novel function of clathrin light chain in promoting endocytic vesicle formation.
Clathrin-mediated endocytosis is a major pathway for uptake of lipid and protein cargo at the plasma membrane. The lattices of clathrin-coated pits and vesicles are comprised of triskelions, each consisting of three oligomerized heavy chains (HC) bound by a light chain (LC). In addition to binding HC, LC interacts with members of the Hip1/R family of endocytic proteins, including the budding yeast homologue, Sla2p. Here, using in vivo analysis in yeast, we provide novel insight into the role of this interaction. We find that overexpression of LC partially restores endocytosis to cells lacking clathrin HC. This suppression is dependent on the Sla2p binding region of LC. Using live cell imaging techniques to visualize endocytic vesicle formation, we find that the N-terminal Sla2p binding region of LC promotes the progression of arrested Sla2p patches that form in the absence of HC. We propose that LC binding to Sla2p positively regulates Sla2p for efficient endocytic vesicle formation. (+info)