The C-terminus of lipoprotein lipase is essential for biological function but contains no domain for glycosylphosphatidylinositol anchoring

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Abstract
In this study we present evidence that the C‐terminus of lipoprotein lipase contains no glycosylphosphatidylinositol addition signal and is therefore not a glycosylphosphatidylinositol‐anchored protein. Furthermore, we present additional evidence that the C‐terminus of lipoprotein lipase is essential for biological function. Flow cytometric analysis and enzyme‐activity monitoring experiments revealed no pool of lipoprotein lipase releasable by phosphatidylinositol‐specific phospholipase present on the membrane of COS cells transfected with the human lipoprotein lipase gene while, in contrast, a heparin‐releasable pool could be demonstrated. [14C]Ethanolamine, a constituent of the glycosylphosphatidylinositol anchor, was not incorporated into lipoprotein lipase during metabolic labeling. C‐terminal deletion mutants were constructed and expressed in COS cells to investigate the presence of glycosylphosphatidylinositol addition signal on the C‐terminus of human lipoprotein lipase (LPL). The specific activities of the mutants M442 [Leu443–Gly448)‐LPL] and M437 [des‐(Cys438–Gly448)‐LPL] were 78% and 59%, respectively, less than the wild type, while the M432 mutant [des‐(Ala433–Gly449)‐LPL] was catalytically inactive. Determination of the stability of the mutants revealed a decreased stability of the M437, compared with wild‐type, whereas M442 showed the same stability. Flow cytometric analysis showed sustained membrane expression for all mutants including the inactive M432 mutant. These results suggest that the C‐terminus of lipoprotein lipase is essential for maintaining intact catalytic activity but is not involved in any posttranslational proteolytic processing, including cleavage of a glycosylphosphatidylinositol addition signal. We therefore conclude that membrane‐binding of the lipase is not mediated by such anchoring.

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