Biology subjects

Zein, H. S.

Publications and source records attributed to Zein, H. S..

3 recordsLinked to original sources

bioRxiv2026

Quaternary structure and activity of glutamate dehydrogenase are regulated by reversible S-palmitoylation and mitochondrial acyl-protein thioesterases.

Glutamate dehydrogenase (GDH) is a key mitochondrial enzyme that catalyzes the reversible oxidative deamination of glutamate to -ketoglutarate, thereby linking amino acid and carbohydrate metabolism. GDH forms catalytically active hexamers and is regulated by various allosteric modulators, including ADP and GTP. Here, we demonstrate that GDH undergoes auto-palmitoylation in the presence of palmitoyl-CoA, leading to a dose-dependent inhibition of enzymatic activity. Using acyl-PEG exchange assays and mass spectrometry, we show that GDH monomers are predominantly mono-palmitoylated, with modification detected at multiple cysteine residues, including Cys55, Cys115, and Cys197, among the six cysteines in the mature enzyme. Blue Native PAGE analysis revealed that palmitoylation disrupts the native hexameric assembly of mammalian GDH, which is organized as a dimer-of-trimers, promoting dissociation into dimers. Importantly, this modification is reversible, as incubation with mitochondrial acyl-protein thioesterases 1 (APT1) and, to a lesser extent, /{beta} hydrolase domain 10 (ABHD10) restores both the hexameric structure and enzymatic activity. The modified Cys55 residues are positioned near the trimer interface, providing a mechanism by which palmitoylation could prevent hexamer formation, whereas Cys115 and 197 may destabilize individual trimers. These findings establish S-palmitoylation as a novel regulatory mechanism for GDH, linking mitochondrial lipid metabolism to the reversible control of a central metabolic enzyme.

biochemistry↗

bioRxiv2026

Membrane contact site resident PTP1B limits superoxide production by suppressing a Syk-Shc1-Phagocyte Oxidase relay.

Phagocytosis is a specialized endocytic process used by macrophages and dendritic cells to engulf particles, which requires coordinated signaling cascades, cytoskeletal remodeling, and assembly of antimicrobial machinery to eliminate pathogens. During Fc {gamma} receptor (Fc{gamma}R)-mediated phagocytosis, dynamic actin depolymerization at the base of the phagocytic cup creates permissive conditions for endoplasmic reticulum-plasma membrane (ER-PM) membrane contact sites (MCS) to form. We demonstrate that the ER-resident protein tyrosine phosphatase PTP1B localizes to newly formed or expanded ER-PM MCS during phagocytosis and dephosphorylates Syk. Using TIRF microscopy with MCS residents, including MAPPER, STIM1, and E-Syts, we show that actin clearance allows ER proteins to approach the plasma membrane. PTP1B colocalizes with Fc{gamma}Rs in actin-cleared zones and physically interacts with Syk, a critical mediator of phagocytic signaling. Loss of PTP1B led to sustained Syk hyperphosphorylation without affecting phagocytosis. However, the PTP1B-deficient cells showed a {asymp}3-fold increase in NADPH oxidase 2 (NOX2)-mediated superoxide production. Using unbiased proteomics, we identified the adapter protein Shc1 as a critical intermediate linking Syk phosphorylation to NOX2 activation. Shc1 phosphorylation during phagocytosis is dependent on Src family kinases and Syk, while genetic ablation of SHC1 reduced superoxide production by {asymp}40%. Proximity ligation assays reveal enhanced Shc1-p47phox interactions in PTP1B-deficient cells during phagocytosis. These findings establish an SFK-Syk-Shc1-NOX2 signaling axis that PTP1B negatively regulates at MCS between the ER and the forming phagosome, providing new mechanistic insights into antimicrobial responses during phagocytosis.

cell biology↗

bioRxiv2025

Acyl-protein thioesterase 1 (LYPLA1) activity promotes the growth of MDA-MB-468 triple-negative breast cancer cells.

Protein S-acylation is a lipid-based, often reversible post-translational modification that can regulate many aspects of protein behavior, including subcellular localization, protein-interactions, and activity. Emerging evidence has identified roles for individual protein acyltransferases encoded by the ZDHHC in cancers, yet the roles of de-S-acylation enzymes are less clear. Recent evidence suggests that acyl-protein thioesterase (APT1)/LYPLA1 can impact epithelial-mesenchymal transition and metastasis. This study integrates patient datasets, CRISPR dependency data, and in vitro assays to find APT1 as a context-dependent vulnerability in triple-negative breast cancer (TNBC). Despite the highest protein abundance in luminal MCF7 cells, basal-like MDA-MB-468 cells exhibited the most prominent specific APT1 activity, reflecting subtype-specific regulation. Inhibition of APT1 with ML348 increased S-acylation of nuclear and mitochondrial proteins without altering global acylation. Functionally, APT1 inhibition reduced cell proliferation while inducing minimal apoptosis, consistent with cytostatic growth arrest. Cell-cycle analysis revealed G1 accumulation and reduced S/G2 transition, linking proteomic changes to impaired replication. These findings establish APT1 as a regulator of TNBC proliferation through dynamic de-S-acylation of cell-cycle and mitochondrial proteins, highlighting it as a potential therapeutic vulnerability in aggressive breast cancers.

cancer biology↗