Biology subjects

Siu, W. S.

Publications and source records attributed to Siu, W. S..

3 recordsLinked to original sources

bioRxiv2026

Clathrin differentially adapts its trimerisation domain during mammalian evolution to traffic the insulin-responsive GLUT4 glucose transporter.

In humans, the CHC22 isoform of clathrin regulates glucose metabolism by trafficking the GLUT4 glucose transporter for intracellular storage in skeletal muscle and release following insulin signalling. Some vertebrate lineages have lost the gene encoding CHC22 but operate the same insulin-stimulated GLUT4 trafficking pathway. Here, we show that species lacking CHC22 exclusively produce an alternatively-spliced form of the universally expressed CHC17 clathrin isoform (CHC17-SAS) with a truncated C-terminus similar to CHC22, expressed predominantly in skeletal muscle. Through its trimerisation domain, CHC17-SAS binds the CHC22-specific adaptor SNX5 that enables CHC22's distinct intracellular function. The 2.3 [A] crystal structure of the CHC22 trimerisation domain demonstrates conservation of the core trimeric fold from CHC17 but differences in electrostatic surface charge that may account for their differential properties. Using GLUT4 translocation assays in HeLa cell models, we show that CHC17-SAS is a functional surrogate for CHC22. Identification of CHC17-SAS resolves the evolutionary conundrum posed by CHC22 absence in some vertebrate lineages, and reveals a common mechanism for mammalian GLUT4 trafficking.

cell biology↗

bioRxiv2024

Suppression of TGF-β/SMAD signaling by an inner nuclear membrane phosphatase complex

Cytokines of the TGF-{beta} superfamily control essential cell fate decisions via receptor regulated SMAD (R-SMAD) transcription factors. Ligand-induced R-SMAD phosphorylation in the cytosol triggers their activation and nuclear accumulation. We determined how R-SMADs are inactivated by dephosphorylation in the cell nucleus to counteract signaling by TGF-{beta} superfamily ligands. We showed that R-SMAD dephosphorylation is mediated by an inner nuclear membrane associated complex containing the scaffold protein MAN1 and the CTDNEP1-NEP1R1 phosphatase. Structural prediction, domain mapping and mutagenesis revealed that MAN1 binds independently to the CTDNEP1-NEP1R1 phosphatase and R-SMADs to promote their inactivation by dephosphorylation. Disruption of this complex led to nuclear accumulation of R-SMADs and aberrant signaling, even in the absence of TGF-{beta} ligands. These findings establish CTDNEP1-NEP1R1 as the elusive R-SMAD phosphatase and reveal the mechanistic basis for TGF-{beta} signaling inactivation and how this process is disrupted by disease-associated MAN1 mutations.

cell biology↗

bioRxiv2022

Regulated Degradation of the Inner Nuclear Membrane Protein SUN2 Maintains Nuclear Envelope Architecture and Function

Nuclear architecture and functions depend on dynamic interactions between nuclear components (such as chromatin) and inner nuclear membrane (INM) proteins. Mutations in INM proteins interfering with these interactions result in disease. However, mechanisms controlling the levels and turnover of INM proteins remain unknown. Here, we describe a mechanism of regulated degradation of the INM SUN domain-containing protein 2 (SUN2). We show that Casein Kinase II and the C-terminal domain Nuclear Envelope Phosphatase 1 (CTDNEP1) have opposing effects on SUN2 levels by regulating SUN2 binding to the ubiquitin ligase Skp/Cullin1/F-Box{beta}TrCP (SCF{beta}TrCP). Upon binding to phosphorylated SUN2, SCF{beta}TrCP promotes its ubiquitination. Ubiquitinated SUN2 is membrane extracted by the AAA ATPase p97 and delivered to the proteasome for degradation. Importantly, accumulation of non-degradable SUN2 results in aberrant nuclear architecture, vulnerability to DNA damage and increased lagging chromosomes in mitosis. These findings uncover a central role of proteolysis in INM protein homeostasis.

cell biology↗