GSK-3–mediated phosphorylation couples ER–Golgi transport and nuclear stabilization of the CREB-H transcription factor to mediate apolipoprotein secretion

• Published in: Molecular biology of the cell Vol. 28; no. 11; pp. 1565 - 1579
• Main Authors: Barbosa, Sónia, Carreira, Suzanne, O’Hare, Peter
• Format: Journal Article
• Language: English
• Published: United States The American Society for Cell Biology 01.06.2017
• Subjects: Amino Acid Motifs; Amino Acid Sequence; Animals; Apolipoproteins - metabolism; Apolipoproteins A; Cell Culture Techniques; Cell Nucleus - metabolism; Chlorocebus aethiops; COS Cells; Cyclic AMP Response Element-Binding Protein - genetics; Cyclic AMP Response Element-Binding Protein - metabolism; Cyclic AMP Response Element-Binding Protein - physiology; Cytoplasm - metabolism; Endoplasmic Reticulum - metabolism; Endoplasmic Reticulum - physiology; Glycogen Synthase Kinase 3 - metabolism; Golgi Apparatus - metabolism; Golgi Apparatus - physiology; Hep G2 Cells; Humans; Lipolysis; Phosphorylation; Secretory Pathway; Transcription Factors - metabolism
• DOI: 10.1091/mbc.e17-01-0075
• ISSN: 1059-1524; 1939-4586

CREB-H, an ER-anchored transcription factor, plays a key role in regulating secretion in metabolic pathways, particularly triglyceride homeostasis. It controls the production both of secretory pathway components and cargoes, including apolipoproteins ApoA-IV and ApoC-II, contributing to VLDL/HDL distribution and lipolysis. The key mechanism controlling CREB-H activity involves its ER retention and forward transport to the Golgi, where it is cleaved by Golgi-resident proteases, releasing the N-terminal product, which traffics to the nucleus to effect transcriptional responses. Here we show that a serine-rich motif termed the P-motif, located in the N-terminus between serines 73 and 90, controls release of the precursor transmembrane form from the ER and its forward transport to the Golgi. This motif is subject to GSK-3 phosphorylation, promoting ER retention, while mutation of target serines and drug inhibition of GSK-3 activity coordinately induce both forward transport of the precursor and cleavage, resulting in nuclear import. We previously showed that for the nuclear product, the P-motif is subject to multiple phosphorylations, which regulate stability by targeting the protein to the SCF Fbw1a E3 ubiquitin ligase. Thus phosphorylation at the P-motif provides integrated control of CREB-H function, coupling intercompartmental transport in the cytoplasm with stabilization of the active form in the nucleus.