title: Serine/threonine kinase 16 and MAL2 regulate constitutive secretion of soluble cargo in hepatic cells.
abstract: MAL2 (myelin and lymphocyte protein 2) is thought to regulate at least two steps in the hepatic apical transcytotic pathway. As vesicle budding and delivery at each step are driven by complex machineries, we predicted that MAL2 participates in several large protein complexes with multiple binding partners. To identify novel MAL2 interactors, we performed split-ubiquitin yeast two-hybrid assays and identified STK16 (serine/threonine kinase 16) as a putative interactor which we verified morphologically and biochemically. As STK16 is a Golgi-associated constitutively active kinase implicated in regulating secretion and because of the massive constitutive secretory capacity of hepatic cells, we tested whether MAL2 and STK16 function in secretion. Expression of a dominant-negative kinase-dead STK16 mutant (E202A) or knockdown of MAL2 impaired secretion that correlated with decreased expression of albumin and haptoglobin. By using 19°C temperature blocks and lysosome deacidification, we determined that E202A expression or MAL2 knockdown did not interfere with albumin synthesis or processing, but led to albumin lysosomal degradation. We conclude that MAL2 and the constitutively active STK16 function to sort secretory soluble cargo into the constitutive secretory pathway at the TGN (trans-Golgi network) in polarized hepatocytes.
authors.affiliation: *Department of Biology, The Catholic University of America, Washington, DC 20064, U.S.A.
authors.forename: Julie G
authors.initials: JG
authors.lastname: In
authors.affiliation: *Department of Biology, The Catholic University of America, Washington, DC 20064, U.S.A.
authors.forename: Anneliese C
authors.initials: AC
authors.lastname: Striz
authors.affiliation: †Department of Regenerative Cardiology, Fundación Centro Nacional de Investigaciones Cardiovasculares Carlos III, Melchor Fernández Almagro 3, E-28029 Madrid, Spain.
authors.forename: Antonio
authors.initials: A
authors.lastname: Bernad
authors.affiliation: *Department of Biology, The Catholic University of America, Washington, DC 20064, U.S.A.
authors.forename: Pamela L
authors.initials: PL
authors.lastname: Tuma
mesh_terms: D000418:Albumins / Q000378:metabolism; D022781:Hepatocytes / Q000201:enzymology / Q000378:metabolism*; D006801:Humans; D008247:Lysosomes / Q000201:enzymology / Q000378:metabolism; D062806:Myelin and Lymphocyte-Associated Proteolipid Proteins / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D021381:Protein Transport; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D020798:Two-Hybrid System Techniques; D021601:trans-Golgi Network / Q000201:enzymology / Q000378:metabolism
chemical_list: D000418:Albumins; C438606:MAL2 protein, human; D062806:Myelin and Lymphocyte-Associated Proteolipid Proteins; D014157:Transcription Factors; D017346:Protein Serine-Threonine Kinases; C418980:STK16 protein, human
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