Serine/threonine kinase 16 and MAL2 regulate constitutive secretion of soluble cargo in hepatic cells

MAL2 (myelin and lymphocyte protein 2) is thought to regulate at least two steps in the hepatic apical transcytotic pathway. Because 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 serine-threonine kinase 16 (STK16) as a putative interactor which we verified morphologically and biochemically. Because 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 19oC 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 in polarized hepatocytes.

Source: BJ Cell - August 1, 2014 Category: Biochemistry Authors: J G. In, A C. Striz, A Bernad, P L. Tuma Tags: BJ Cell Source Type: research